feat(ios): paginate older turns with infinite scroll

[DatScreamer]

Closes part of #306
Depends on #307 (land after it so the paged cursor behaves predictably).

Problem

iOS conversation history loading was manual (a "Load earlier messages" button), page size 5, and the "Latest" jump button could be consumed just by stopping a decelerating ScrollView.

Changes (ConversationView, AppModel, TranscriptTurn, Settings, tests)

• Scroll-triggered prefetch: tracks visible turn IDs and loads the next older page as the user approaches the top, debounced per cursor so concurrent prefetches collapse.
• Page size 5 → 20; loadOlderTurns now returns whether a page loaded.
• "Latest" re-routed through semantic ScrollPosition + a UIKit scroll-view resolver so the jump executes even while momentum is decelerating.
• Merged exploration rows anchored to their newest constituent turn so prepending older pages keeps scroll identity stable.
• Auto-collapse conversations past 200 items.

Scope

iOS only; the conversation surface is shared across all runtimes, so this improves every provider (codex, claude, pi, local-studio, opencode).

Parity note

Android still uses a manual "Load earlier messages" button (ConversationScreen.kt). The Rust metadata-read fix (#307) benefits both; the infinite-scroll UX is iOS-only in this change. Android parity is a follow-up.

Tests

• 3 new PerformanceHelpersTests: auto-collapse, prefetch-before-cache-edge, exploration identity.

Verification

• make ios-sim-fast

[DatScreamer]

Depends on the server-side pagination from 0xSero/alleycat#49 (issue 0xSero/alleycat#48), pinned via #307's Cargo.lock bump.

[0xSero]

#319 has merged (0824605d) and rewrote ConversationView.swift substantially, so this will need a rebase. The Rust half of #306 is now in — include_turns=false is authoritative and page size is 20 — so what remains here is the scroll integration itself: prefetch, the "Latest" jump, and auto-collapse. Still wanted.

[DatScreamer]

This PR is superseded by #331 (feat(home): infinite scroll + pull-to-refresh for the home sessions list). The conversation older-turns infinite scroll work is now carried on the feat/home-sessions-infinite-scroll branch (commit d3ab5a22), which this PR's home branch already contains. Keeping both open would race on the same AppModel.swift paging code and Rust cursor state. Recommend closing this one in favor of #331.