perf(main): PTY replay buffers copy the whole cap on every chunk and retain sliced-string parents #726

Symptom

Every PTY chunk from every live session costs the main process a copy of the whole replay buffer, and the retained buffer can pin up to twice its nominal cap.

src/main/sessionManager.ts keeps two per-session replay buffers as plain strings — terminalBuffers (cap 256 KiB) and agentPtyBuffers (cap 512 KiB) — and appends with appendCappedBuffer (~L404):

let next = prev + data          // ConsString
...
const firstCode = next.charCodeAt(startIdx)   // forces a flatten: fresh (cap + chunk) copy
next = next.slice(startIdx)     // SlicedString that RETAINS the flattened parent

Once a buffer is full (which every agent TUI reaches within minutes — full-screen repaints), each chunk allocates ~cap + chunk bytes and the stored value is a slice whose parent is that whole allocation. With 20 sessions emitting repaint chunks continuously this is on the order of 100+ MB/s of short-lived string allocation on the main thread, i.e. steady GC pressure of the #390 class, and the retained per-session buffer is cap + last chunk, not cap.

Evidence

The 2026-09-03 main-process heap snapshot (--owners / string survey, see #720 for the run) contains six identical 1.8 MB strings beginning with a docker [+] Building ANSI repaint and ~10 identical ~1 MB Claude screen-frame strings: PTY chunks larger than the cap become the flattened parent that the stored slice keeps alive, one per session that saw them. [debug-retention] and heartbeat records for the run show main eventLoop.meanMs up to 1591 during heavy multi-session output.

Expected

Appending a chunk should be amortised O(chunk), not O(cap); the retained size should never exceed the cap; and a chunk larger than the cap should be stored as a real copy of its tail, not a slice over the oversized chunk (#321 lesson: V8 slices keep their parent alive).

Proposed shape

Replace the string-per-session with a small chunk-list buffer (append pushes the chunk and drops whole oldest chunks while over the cap; read joins on attach, which is rare). Both buffers keep their exact contract: bytes accumulate from process start, attachTerminal / attachAgentPty return the current tail atomically, and the caps are unchanged.

Impact

Main-process GC churn proportional to (sessions × chunk rate × cap); up to 2× cap retained per session; oversized chunks retained in full.

Refs #390, #321, #103, #327.