Forum posts and comments cannot be edited: Desktop discards the kind:40003 edits the relay already accepts

## What's missing

There is no way to edit a forum post (kind:45001) or a forum comment (kind:45003) in Buzz.
Forum cards expose a Delete action and nothing else, so the only way to fix a typo in a
long-form post is to delete it and repost, which loses the event id that every reply,
reaction and mention already points at.

This is not a relay limitation. The relay already accepts and stores the edit. Desktop
fetches it and then throws it away.

## Evidence: the relay accepts it today

Measured on a private, ephemeral forum channel:

| Event | Kind | Result |
|---|---|---|
| `2d65a211c38e1f5bb9c552d6a68ded90f6ecd1b8b84236da8ab21a86d439484a` | 45001 | post created |
| `e85cda57c2d7271c9a856f9b08c306781524263b735f90f8e3bd2fcd93f03b76` | 40003 | edit of the post, `e` tag -> the 45001, **accepted and stored** |
| `e68aa989f4dd272a9f18bf31f5368326692d31af0187f89a28a86514497b6ebc` | 45003 | comment created |
| `b1efa0248076c7acfba690efd158075a659eb97ea91bae8fbed32fedbf55eea2` | 40003 | edit of the comment, **accepted and stored** |

After both edits, reading the 45001 still returns the original content.

## Why it works at the write layer

`validate_edit_ownership` (`crates/buzz-relay/src/handlers/ingest.rs:920`) checks only that
the signer is the target's effective author (or the owning human of the authoring agent) and
that the target lives in the same channel. It never inspects the target's kind. Compare
`validate_forum_vote_target` in the same file (`:1001`), which explicitly requires
`KIND_FORUM_POST` or `KIND_FORUM_COMMENT` at `:1034`. The edit validator has no equivalent
gate, so a 40003 aimed at a 45001 is valid by construction.

`build_edit` (`crates/buzz-sdk/src/builders.rs:389`) is likewise kind-agnostic on the target.

## Where it stops

1. `desktop/src-tauri/src/commands/messages/forum.rs` does query kind:40003 events for the
rows it returns (`:183`, `:238`), but only to compute link-preview suppression (`:122`).
The edit's content is never applied to the message.
2. `get_forum_thread` (`forum.rs:253`) explicitly filters kind:40003 out of the reply list.
3. No edit affordance exists in the UI. `ForumPostCard.tsx:113` and `ForumThreadPanel.tsx:107`
and `:254` render `DeleteActionMenu` and nothing else.

Note that the data is already delivered: the CLI thread filter
(`crates/buzz-cli/src/commands/messages.rs:439`) lists 40003 among the reply kinds, and the
edit event does come back alongside the root. This is a folding gap, not a fetching gap.

## Why it matters

Forum channels are the long-form surface (`VISION.md:19`, `:28`). Long-form is exactly where
a post is a living document: correcting a spec, fixing a broken link, adding acceptance
criteria after discussion. Chat messages (kind:9) can be edited; the surface built for text
that people actually revise cannot.

Repost-and-delete is not equivalent. It breaks every `e` tag pointing at the old id: replies
are orphaned, reactions are lost, and any agent that stored the id now references a tombstone.

## Suggested fix

Client-side only. No relay change, no new event kind, no spec change.

1. Fold kind:40003 content onto the target in `forum_message_from_event`, the same way the
chat path already folds edits, and stop discarding 40003 at `forum.rs:253`.
2. Add an Edit action beside Delete in `DeleteActionMenu` (or a shared post-action menu) for
posts and comments the current identity authored.
3. Render the usual "edited" marker for parity with chat.

## Related

- #2990 — no way to edit a published issue body (kind:1621). Same family: content revision
was built for kind:9 and nothing else.
- #6388 — kind:9 edits are accepted but `get`/`history` still return the original.

## Environment

Read against `main` at commit `cd0d33f0`, ahead of `desktop-v0.5.17`.