[schiste]

Summary

iOS app performance: main-thread output buffering, diagnostics log trimming, renderer leaks

Problem: Three buffers did whole-buffer work on the main actor once they were
full.

• RemoteTerminalOutputStore rebuilt committed + pending and re-trimmed
  the 200 KB tail on every 16 ms flush, even while the rich renderer was
  on screen and nothing read the text.
• DiagnosticsLog, at its 8000-entry cap, mapped every category, removed
  index 0 and forced a full JSONL rewrite for each new entry. Verbose
  logging is on by default, so this ran constantly.
• The renderer engine kept each tab's replay buffer (up to 400 KB) after
  the playback was built from it, although nothing reads it again.
• The diagnostics viewer filtered all entries twice per body.

Decision: - The output store appends in place and trims only after 32 KB of
slack. visibleOutput still returns exactly the retained cap.

• DiagnosticsRetentionPolicy gains an entryTarget: over 8000 entries the
  log trims to 7000 in one batch, and removals happen in a single pass.
• ensurePlayback frees the replay once the playback exists, and the
  diagnostics list filters once per body.

Rationale: All three are amortisation changes with no visible behaviour change:
the same tails are retained and displayed, and the costs move from every
frame or record to once per batch.

Changes

Checklist

• pnpm quality:prepush passes, or pnpm quality:prepush:full for high-impact changes
• cargo test --workspace passes in apps/chau7-next (if macOS app changes)
• swift test --package-path apps/chau7-ios/Chau7Core passes (if Swift changes)
• New tests added for new behavior
• Documentation updated (README, CHANGELOG.md if user-facing)

Test plan

xcodebuild test, Chau7RemoteApp scheme, iPhone 17 Pro simulator: 129
tests, 0 failures. New tests cover the newest tail surviving amortised
trims, batch trimming to the target, no trim at the cap, and a target
above the cap failing closed. The replay release has no direct test,
because the engine actor is not in the host-less test bundle.

Commits:

• e6f5030 perf(ios): stop per-frame full-buffer copies on the main actor

Opened by aethyme broker push as a draft.