feat(chain): remove channels, add social feed (Phase A of v2.0.0)

Replaces the channel/membership model with a VK/Twitter-style feed:
public posts, follow graph, likes. Views are deliberately off-chain
(counted by the hosting relay, Phase B).

Removed
  - EventCreateChannel, EventAddMember
  - CreateChannelPayload, AddMemberPayload, ChannelMember
  - prefixChannel, prefixChanMember
  - chain.Channel(), chain.ChannelMembers()
  - node/api_channels.go
  - GetChannel, GetChannelMembers on ExplorerQuery

Added
  - Events: CREATE_POST, DELETE_POST, FOLLOW, UNFOLLOW, LIKE_POST, UNLIKE_POST
  - Payloads: CreatePostPayload, DeletePostPayload, FollowPayload,
    UnfollowPayload, LikePostPayload, UnlikePostPayload
  - Stored shape: PostRecord (author, size, hash, hosting relay, timestamp,
    reply/quote refs, soft-delete flag, fee paid)
  - State prefixes: post:, postbyauthor:, follow:, followin:, like:, likecount:
  - Queries: Post(), PostsByAuthor(), Following(), Followers(),
    LikeCount(), HasLiked()
  - Cached like counter via bumpLikeCount helper

Pricing
  - BasePostFee = 1000 µT (aligned with MinFee block-validation floor)
  - PostByteFee = 1 µT/byte of compressed content
  - Total fee credited in full to HostingRelay pub (storage compensation)
  - MaxPostSize = 256 KiB

Integrity
  - CREATE_POST validates content_hash length (32 B) and size range
  - DELETE_POST restricted to post.Author
  - Duplicate FOLLOW / LIKE rejected
  - reply_to and quote_of mutually exclusive

Tests
  - TestFeedCreatePost: post stored, indexed, host credited
  - TestFeedInsufficientFee: underpaid post is skipped
  - TestFeedFollowUnfollow: follow graph round-trips via forward + inbound indices
  - TestFeedLikeUnlike: like toggles with dedup, counter stays accurate
  - TestFeedDeletePostByOther: non-author deletion rejected

This is Phase A (chain-layer). Phase B adds the relay feed-mailbox
(post bodies + gossipsub) and HTTP endpoints. Phase C adds the client
Feed tab.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

blockchain/types.go

@@ -11,8 +11,6 @@ type EventType string
 
 const (
 	EventRegisterKey    EventType = "REGISTER_KEY"
-	EventCreateChannel  EventType = "CREATE_CHANNEL"
-	EventAddMember      EventType = "ADD_MEMBER"
 	EventOpenPayChan    EventType = "OPEN_PAY_CHAN"
 	EventClosePayChan   EventType = "CLOSE_PAY_CHAN"
 	EventTransfer       EventType = "TRANSFER"
@@ -37,6 +35,17 @@ const (
 	EventMintNFT         EventType = "MINT_NFT"          // mint a new non-fungible token
 	EventTransferNFT     EventType = "TRANSFER_NFT"      // transfer NFT ownership
 	EventBurnNFT         EventType = "BURN_NFT"          // burn (destroy) an NFT
+
+	// ── Social feed (v2.0.0) ──────────────────────────────────────────────
+	// Replaces the old channel model with a VK/Twitter-style timeline.
+	// Posts are plaintext, publicly readable, size-priced. Bodies live in
+	// the relay feed-mailbox; on-chain we only keep metadata + author.
+	EventCreatePost      EventType = "CREATE_POST"       // author publishes a post
+	EventDeletePost      EventType = "DELETE_POST"       // author soft-deletes their post
+	EventFollow          EventType = "FOLLOW"            // follow another author's feed
+	EventUnfollow        EventType = "UNFOLLOW"          // unfollow an author
+	EventLikePost        EventType = "LIKE_POST"         // like a post
+	EventUnlikePost      EventType = "UNLIKE_POST"       // remove a previous like
 )
 
 // Token amounts are stored in micro-tokens (µT).
@@ -64,6 +73,31 @@ const (
 	// MinContactFee is the minimum amount a sender must pay the recipient when
 	// submitting an EventContactRequest (anti-spam; goes directly to recipient).
 	MinContactFee uint64 = 5_000 // 0.005 T
+
+	// ── Feed pricing (v2.0.0) ─────────────────────────────────────────────
+	// A post's on-chain fee is BasePostFee + bytes_on_disk × PostByteFee.
+	// The fee is paid by the author and credited in full to the hosting
+	// relay (the node that received POST /feed/publish and stored the body).
+	// Size-based pricing is what aligns incentives: a 200-byte tweet is
+	// cheap, a 256 KB video costs ~0.26 T — node operators' storage cost
+	// is covered.
+	//
+	// Note: BasePostFee is set to MinFee (1000 µT) because chain-level block
+	// validation requires every tx's Fee ≥ MinFee. So the true minimum a
+	// post can cost is MinFee + size × PostByteFee. A 0-byte post is
+	// rejected (Size must be > 0), so in practice a ~50-byte text post
+	// costs ~1050 µT (~$0.001 depending on token price).
+	BasePostFee    uint64 = 1_000 // 0.001 T flat per post — aligned with MinFee floor
+	PostByteFee    uint64 = 1     // 1 µT per byte of stored content
+
+	// MaxPostSize caps a single post's on-wire size (text + attachment, post
+	// compression). Hard limit — node refuses larger envelopes to protect
+	// storage and bandwidth.
+	MaxPostSize uint64 = 256 * 1024 // 256 KiB
+
+	// LikeFee / FollowFee / UnlikeFee / UnfollowFee / DeletePostFee all use
+	// MinFee (1000 µT) — standard tx fee paid to the validator. No extra
+	// cost; these events carry no body.
 )
 
 // Transaction is the atomic unit recorded in a block.
@@ -90,11 +124,66 @@ type RegisterKeyPayload struct {
 	X25519PubKey string `json:"x25519_pub_key,omitempty"` // hex Curve25519 key for E2E messaging
 }
 
-// CreateChannelPayload is embedded in EventCreateChannel transactions.
-type CreateChannelPayload struct {
-	ChannelID string `json:"channel_id"`
-	Title     string `json:"title"`
-	IsPublic  bool   `json:"is_public"`
+// ── Feed payloads (v2.0.0) ─────────────────────────────────────────────────
+
+// CreatePostPayload is embedded in EventCreatePost transactions. The body
+// itself is NOT stored on-chain — it lives in the relay feed-mailbox keyed
+// by PostID. On-chain we only keep author, size, hash, timestamp and any
+// reply/quote reference for ordering and proof of authorship.
+//
+// PostID is computed client-side as hex(sha256(author || content_hash || ts)[:16])
+// — same scheme as envelope IDs. Clients include it so the relay can store
+// the body under a stable key before the chain commit lands.
+//
+// HostingRelay is the node pubkey (Ed25519 hex) that accepted the POST
+// /feed/publish call and holds the body. Readers resolve it via the chain
+// and fetch the body directly from that relay (or via gossipsub replicas).
+// The fee is credited to this pub.
+//
+// QuoteOf / ReplyTo are mutually exclusive; set at most one. ReplyTo makes
+// the post a reply in a thread; QuoteOf creates a link/reference block.
+type CreatePostPayload struct {
+	PostID       string `json:"post_id"`
+	ContentHash  []byte `json:"content_hash"`            // sha256 of body-bytes, 32 B
+	Size         uint64 `json:"size"`                    // bytes on disk (compressed)
+	HostingRelay string `json:"hosting_relay"`           // hex Ed25519 of storing node
+	ReplyTo      string `json:"reply_to,omitempty"`      // parent post ID
+	QuoteOf      string `json:"quote_of,omitempty"`      // referenced post ID
+}
+
+// DeletePostPayload — author soft-deletes their own post. Stored marker
+// lets clients hide the post; relay can GC the body on the next sweep.
+type DeletePostPayload struct {
+	PostID string `json:"post_id"`
+}
+
+// FollowPayload / UnfollowPayload — follow graph. tx.From = follower,
+// tx.To = target. No body.
+type FollowPayload struct{}
+type UnfollowPayload struct{}
+
+// LikePostPayload / UnlikePostPayload — per-post like indicator. tx.From
+// = liker. The counter is derived on read.
+type LikePostPayload struct {
+	PostID string `json:"post_id"`
+}
+type UnlikePostPayload struct {
+	PostID string `json:"post_id"`
+}
+
+// PostRecord is what we store on-chain under post:<postID>. Consumers of
+// PostsByAuthor / query endpoints decode this.
+type PostRecord struct {
+	PostID       string `json:"post_id"`
+	Author       string `json:"author"`                  // hex Ed25519
+	ContentHash  []byte `json:"content_hash"`
+	Size         uint64 `json:"size"`
+	HostingRelay string `json:"hosting_relay"`
+	ReplyTo      string `json:"reply_to,omitempty"`
+	QuoteOf      string `json:"quote_of,omitempty"`
+	CreatedAt    int64  `json:"created_at"`              // unix seconds (tx timestamp)
+	Deleted      bool   `json:"deleted,omitempty"`
+	FeeUT        uint64 `json:"fee_ut"`                  // total fee paid
 }
 
 // RegisterRelayPayload is embedded in EventRegisterRelay transactions.
@@ -241,24 +330,6 @@ type BlockContactPayload struct {
 	Reason string `json:"reason,omitempty"`
 }
 
-// ChannelMember records a participant in a channel together with their
-// X25519 public key. The key is cached on-chain (written during ADD_MEMBER)
-// so channel senders don't have to fan out a separate /api/identity lookup
-// per recipient on every message — they GET /api/channels/:id/members
-// once and seal N envelopes in a loop.
-type ChannelMember struct {
-	PubKey       string `json:"pub_key"`        // Ed25519 hex
-	X25519PubKey string `json:"x25519_pub_key"` // optional; empty if member hasn't registered
-	Address      string `json:"address"`
-}
-
-// AddMemberPayload is embedded in EventAddMember transactions.
-// tx.From adds tx.To as a member of the specified channel.
-// If tx.To is empty, tx.From is added (self-join for public channels).
-type AddMemberPayload struct {
-	ChannelID string `json:"channel_id"`
-}
-
 // AddValidatorPayload is embedded in EventAddValidator transactions.
 // tx.From must already be a validator; tx.To is the new validator's pub key.
 //