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>

cmd/node/main.go

@@ -854,8 +854,6 @@ func main() {
 		GetNFT:            chain.NFT,
 		GetNFTs:           chain.NFTs,
 		NFTsByOwner:       chain.NFTsByOwner,
-		GetChannel:        chain.Channel,
-		GetChannelMembers: chain.ChannelMembers,
 		Events:           sseHub,
 		WS:               wsHub,
 		// Onboarding: expose libp2p peers + chain_id so new nodes/clients can