perf(terminal): append output without replaying retained history

[rynfar]

Long terminal sessions repeatedly rebuilt retained history, replayed whole buffers when retention rolled over, and allocated temporary buffers for snapshots.

Port five upstream terminal fixes (#9663, #9703, #9707, #9748, and #7680). Server history now appends incrementally and retains at most 5,000 lines or 8 MiB. Client output uses chunked storage and an append cursor, Ghostty reuses snapshot buffers safely across WebAssembly memory growth, and PTYs advertise truecolor when COLORTERM is unset. Preserve Pylon’s terminal command ordering and adapt mobile’s existing string-buffer boundary without changing its native ABI.

Validation: 111 focused tests passed across server terminal management, both PTY adapters, shared terminal state, real Ghostty WebAssembly, and mobile terminal menus. Typechecks passed for server, client-runtime, web, and mobile; changed-file lint passed.

Integrated Chromium verification used a real server PTY and Ghostty renderer with 5,300 output lines. Both baseline and candidate retained the latest 5,000-line tail (524,264 characters in this fixture); the candidate reported COLORTERM=truecolor where the baseline was unset. This verifies retained output, rendering, and truecolor behavior; it is not a measured performance benchmark or native mobile run.

Client verification evidence

terminal-after.webm

Model: GPT-6 Astra. Harness: Codex.

[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	13.6 KiB	13.6 KiB	−19 B (−0.1%)	15.1 KiB	✅
Codex	Thread snapshot wire	7.1 KiB	7.1 KiB	−14 B (−0.2%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	6.6 KiB	6.5 KiB	−5 B (−0.1%)	7.8 KiB	✅
Codex	Live turn WebSocket decoded	56.5 KiB	56.5 KiB	0 B (0.0%)	66.4 KiB	✅
Codex	Live turn messages	8	8	0 (0.0%)	21	✅
Claude	Total thread wire	13.8 KiB	13.6 KiB	−129 B (−0.9%)	15.1 KiB	✅
Claude	Thread snapshot wire	7.1 KiB	7.1 KiB	+8 B (+0.1%)	7.3 KiB	✅
Claude	Live turn WebSocket wire	6.7 KiB	6.6 KiB	−137 B (−2.0%)	7.8 KiB	✅
Claude	Live turn WebSocket decoded	58.1 KiB	57.3 KiB	−840 B (−1.4%)	66.4 KiB	✅
Claude	Live turn messages	8	8	0 (0.0%)	21	✅

Baseline: c966815 · PR result: 311e7c5 · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 111.2 KiB
• Claude decoded thread snapshot: 111.9 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.