bug: Large conversations render unbounded (crash / blank surface) on iOS; older history loads via a manual button

[DatScreamer]

Reproduction:

1. Open a conversation with many turns (e.g. a large OpenCode or codex thread).
2. On iOS, the app can block the main thread until the watchdog kills it (0x8BADF00D), or render a blank conversation surface.
3. Scrolling up to read older messages requires tapping "Load earlier messages" repeatedly, with small (5-turn) pages.

Root cause:
The client issues metadata-only thread/read requests (include_turns=false) in the shared multi-runtime resume/fallback path. Some bridges — including the opencode bridge — return the full turn archive even for metadata-only reads. The client merged all embedded turns unconditionally, so a 5-turn page could silently become hundreds of UI items rendered at once, exceeding SwiftUI's layout budget and blocking the main thread.

Additionally, iOS had no scroll integration for history loading: a manual button, page size 5, and a "Latest" jump that could be swallowed by a decelerating ScrollView.

Fix (two PRs):

• #PR1: honor include_turns=false so metadata reads stay bounded (shared Rust, all runtimes), plus pin the alleycat fork's pagination commits.
• #PR2: replace the manual button with scroll-triggered infinite scroll, larger pages, a working "Latest" jump, and auto-collapse for large conversations (iOS).

[DatScreamer]

Fix split across two PRs:

• fix(thread): keep metadata thread reads paginated #307 fix(thread): keep metadata thread reads paginated — shared Rust, runtime-agnostic: honors include_turns=false so metadata reads stay bounded; pins the alleycat fork's pagination commits.
• feat(ios): paginate older turns with infinite scroll #308 feat(ios): paginate older turns with infinite scroll — iOS: scroll-triggered older-page prefetch, page size 5→20, working "Latest" jump, auto-collapse past 200 items.

[DatScreamer]

Server-side dependency now upstreamed: 0xSero/alleycat#49 (closes 0xSero/alleycat#48) — pins the opencode bridge pagination + session enumeration that #307 consumes.

[0xSero]

Partially fixed in #319 (merged as 0824605d).

Fixed: the unbounded-render root cause. c153d1e5 makes include_turns=false authoritative, so a compatibility bridge returning a full thread archive for a metadata-only read can no longer replace a bounded page with hundreds of UI items. Turn page size also went 5 → 20, which removes the "tap Load earlier 40 times" problem — safe only because of the include_turns fix above.

Still open: the iOS scroll integration from #308 — scroll-triggered prefetch, a working "Latest" jump, and auto-collapse. That PR needs a rebase onto post-#319 ConversationView.swift, which was substantially rewritten.