refactor(chat): drop the deprecated new_messages overlay

[bmc08gt]

ChatUpdate.new_messages (field 2) is marked [deprecated = true] in flipcash2-protobuf-api, superseded by the sequenced, gap-detectable events batch (messaging.v1.EventBatch, field 6). The decode path kept it as an additive overlay so a message arriving only there was never dropped. The backend now sends events and leaves new_messages empty, so the overlay only costs a second pass over an empty batch and an enum case that every switch over ConversationStreamEvent has to carry.

Nothing is lost

.chatEvents already covers what the .newMessages arm did, in both places that handled it:

• Store — the .newMessages arm bumped last activity to the newest message by id. applyChatEvents does the same, restricted to .sent mutations, so an edit or delete can't move a feed row on its original low id. That restriction is the correct behaviour, not a regression.
• Controller — the .newMessages persist arm read the analytics watermark, reconciled pending sends, wrote messages, counted receipts, and scheduled a gap catch-up on a failed write. The .chatEvents arm does all of it, plus the cursor write that makes the messages and the advanced frontier land atomically.

upsertConversationMessages, which only the removed arm called on this path, is still used by the send, load-older, and mutation paths.

Tests

Ported, because they cover ordering, receipt wiring, and backlog handling rather than the deprecated field:

• ConversationStreamEventDecodeTests — the decode-ordering and messages-plus-typing cases now build events fixtures.
• ConversationControllerTests and ConversationReceiptWiringTests — mock.emit now goes through a new ConversationStreamEvent.sent(_:in:) test helper that wraps messages as one contiguous run of .sent mutations. The reconnect-replay dedup still holds: the second emit reads a watermark that already includes the message, so it isn't re-credited.

Deleted, because they only existed to cover the overlay:

• the basic new_messages decode test (chatEventsDecode covers decoding events),
• chatEventsAndNewMessagesAdditive and emptyEventsKeepsNewMessages, both migration-window gates,
• ConversationStoreTests.newMessagesBumpsActivity — chatEventsBumpsActivity already asserts the same last-activity behaviour over .chatEvents, so re-expressing it would have duplicated an existing test.

Notes

Stacked on #756, which is itself the top of the NSE preload stack. The commit doesn't depend on that stack — the only thing those PRs change in the files touched here is three import FlipcashStore lines — so it can be re-parented onto main if the stack is going to sit.

A matching removal is queued for Android (EventStreamDelegate.applyUpdate, which uses the field as an exclusive fallback rather than an additive overlay). The two are independent.