fix(client): read() chat target, dead unsend() guard, i64 fetch cursor

[nezumi0627]

Summary

Three client-facing bugs, each verified with a failing call before the fix:

1. TalkMessage.read() marked the wrong chat as read for received group messages

chatMid: this.isMyMessage ? this.to.id : this.from.id

For a group/room message received from someone else, from.id is the sender's USER mid, but sendChatChecked's chatMid must be the chat id — so .read() targeted a (usually non-existent) 1:1 chat instead of the group. The neighboring reply()/send() already branch on toType, which makes this look like an oversight.

Now: GROUP/ROOM → always to.id; USER chats keep the previous behavior (counterpart mid).

2. SquareMessage.unsend() / SquareThreadMessage.unsend() ownership guard was dead code

isMyMessage is an async method on square messages, but both unsend() implementations tested it as a property:

if (!this.isMyMessage) { throw new TypeError(...) }

this.isMyMessage is a function object — always truthy — so the guard never fired and unsend() proceeded for anyone's message. Now awaited properly. (TalkMessage.unsend() is unaffected; its isMyMessage is a getter.)

3. Message-fetcher pagination cursor lost i64 precision + crashed on empty pages

createMessageFetcher().fetch() did:

• parseInt(lastMessage.id) — LINE message ids are ~19-digit values beyond Number.MAX_SAFE_INTEGER, so the next page's cursor was silently rounded, skipping/duplicating messages. Now BigInt(lastMessage.id) (the struct writer already supports bigint I64).
• messages.at(-1)! — threw TypeError when a fetch returned an empty array (end of history). Empty pages now return [] with the cursor left unchanged.

Testing

New tests: talk.test.ts (group read targets chat mid; 1:1 unchanged), square.test.ts (unsend of another member's message throws TypeError without calling unsendMessage), fetcher.test.ts (2^53+1 cursor round-trips losslessly; empty page returns [] without throwing).

Full suite: deno test --allow-all — 218 passed, 0 failed.