Release 0.0.166: reliable JSONL logging (#86) + compaction-failure recovery (#88)

[justrach]

TUI/harness fixes for two backend correctness bugs, plus the GUI version bump to 0.0.166. All changes are in src/main.zig (the Zig harness); the GUI ships the embedded harness, so it gets the fix via the version bump.

#86 — HttpConnectionClosing retries give up turn & corrupt trajectory log

• JSONL corruption (root cause). Tracer.write / Trajectory.writeLocked serialized JSON directly into the shared buffered file writer and catch return on a partial serialize — leaving a half-record plus leftover buffer bytes that the next record concatenated onto (exactly the reported {"text":"You ar{"kind":… garbage). New writeJsonLine() builds the whole line in memory first, so a failed write emits nothing and the file stays valid JSONL. Both writers route through it.
• Flaky retries. Transport flakes (HttpConnectionClosing, resets, truncated TLS) retried with zero delay — 3 instant re-fails against a just-closed keep-alive. Added a 250ms·2ⁿ backoff (capped 2s, Esc-cancellable) and one more attempt.
• Error preservation. On network give-up we now set last_api_error to the real transport error (was stale/null), and write an adjacent {"kind":"turn_error", "parent":<turn>, "detail":…} archive record so a failed turn's reason survives instead of just ok:false.

#88 — compaction can fail/skip silently, leaving huge context

• Root cause. Once context overflows the window, compact()'s own summarization request overflows too and fails — and that failure was swallowed (error.ApiError => {}), so the session wedged and every tiny follow-up failed at the same token count.
• Fix. New compactOrRecover() surfaces the failure (no more silent skip) and, as a backstop, emergency-trims history at a safe user-turn boundary (emergencyCutIndex/cleanUserTurn, which correctly refuses an anthropic tool_result-only user message as a cut point). Compaction is now also attempted after a failed turn (context-overflow recovery), not only after successful ones.

Testing

• 98/98 unit tests pass (4 new: writeJsonLine, cleanUserTurn, emergencyCutIndex incl. the tool_result-boundary case). zig build + zig build test green.
• Runtime-verified: a real failing turn produces 4/4 valid trajectory records + 2/2 valid trace records, with the new turn_error record carrying the preserved error detail.

Not in this PR (scope)

• deepseek-v4-pro: no working reasoning-budget control (only all-or-nothing thinking) #89 (deepseek reasoning-budget) is a gateway-side change (gateway.codegraff.com), not this harness — the harness already sends reasoning_effort and degrades gracefully when rejected.
• Make TUI Thinking messages collapsible #75 (thinking collapse) and TUI: auto-copy highlighted text on mouse selection #85 (mouse-select auto-copy) need an alt-screen TUI; the REPL is inline scrollback and can't retro-collapse output or capture mouse without breaking native terminal selection.
• Fix segmented button borders to render on all four sides #74 / GUI bug: image attachment indicator disappears when navigating prompt history #78 / Add GUI /ultracode on/off toggle choices with keyboard selection #79 already shipped in 0.0.165.

Follow-up (not part of the reported bug)

Agent.emit / guiEmit (the --json/GUI stdout writers) still use the old hand-rolled Stringify → writeByte('\n') → flush pattern straight into shared g_out, so they carry the same #86 torn-write risk. Left out here because it's a hot path (printDelta → emit per streamed chunk) — doing it right needs a shared scratch buffer under g_gui_mu, not a per-call allocation. Worth a focused follow-up.

Fixes #86. Fixes #88.