Phase 2e: rich comments — optional ref_post_id with signed preview

A comment can now reference a separate Post that carries the full body
(long text, attachments, rich formatting). The inline comment's `content`
becomes a short preview string; the referenced post propagates through the
normal CDN and readers fetch it lazily when rendering the expanded view.

Type change:
- `InlineComment` gains `ref_post_id: Option<PostId>` (#[serde(default)]).
  When None, `content` is the full comment text (v0.6.1 shape — unchanged on
  the wire). When Some, `content` is the preview.

Signature binding:
- `crypto::sign_comment` / `verify_comment_signature` now take
  `ref_post_id: Option<&PostId>`. The signed digest appends
  `b"ref:" || ref_post_id` only when a ref is present, so plain comments
  produce the same digest as the v0.6.1 scheme and remain verifiable
  without a migration. When a ref is present the signature binds it, so a
  peer can't strip or swap the reference without re-signing.

Storage:
- `comments` table gets a `ref_post_id BLOB` column (nullable). Added to
  both the CREATE TABLE statement and a conditional ALTER TABLE migration
  so upgraded DBs pick it up automatically.
- `store_comment`, `get_comments`, `get_comments_with_tombstones` read and
  write the column.

Node API:
- `comment_on_post` stays as the plain-comment entry point (calls the
  inner helper with `ref_post_id = None`).
- New `comment_on_post_with_ref(post_id, preview, ref_post_id)` for rich
  comments. Both share a single inner helper that signs, stores, and
  propagates via BlobHeaderDiff.

connection.rs BlobHeaderDiff handler passes `comment.ref_post_id.as_ref()`
to the signature verify so forged or rewritten refs are rejected.

Tests: new crypto test asserting the signature binds ref_post_id (strip /
swap / drop all fail); new storage test asserting ref_post_id roundtrips
through live + tombstone reads. 116 / 116 core tests pass.

Client-side UX (pulling the ref post on expand, composing rich comments)
is frontend work that will land with the next UI iteration.

crates/core/src/connection.rs

@@ -6117,6 +6117,7 @@ impl ConnectionManager {
                             &comment.content,
                             comment.timestamp_ms,
                             &comment.signature,
+                            comment.ref_post_id.as_ref(),
                         ) {
                             continue; // Skip forged comments
                         }