refactor(websocket): extract setupNewPtyEntry phases into helpers

[almogdepaz]

Extracts the 478-LOC setupNewPtyEntry god function into four module-level helpers driven by a shared PtyEntryContext. Pure refactor, zero behavior changes.

Helpers

• waitForResizeSettle(ctx, initialSize) — pre-snapshot client-resize debounce loop. Returns settled size or null on bail.
• waitForOutputQuiescence(ctx, pendingSize) — applies settled dims, then observes broker output until quiet (so SIGWINCH redraw lands in the snapshot, not the live replay). Returns final applied size or null.
• sendSnapshotPrefill(ctx, appliedSize, prefillMode) — fetches broker snapshot, chunked send to viewer, returns prefillSeq for live-subscribe sinceSeq.
• subscribeWithCoalescing(ctx, prefillSeq) — live data sub with adaptive coalescing + lifecycle sub + teardown wiring.

Context

interface PtyEntryContext {
  readonly entry: PtyEntry;
  readonly session: string;
  readonly ws: WebSocket;
  readonly backend: SessionBackend & PtyBackendMethods;
  readonly requestedSize: { current: { cols: number; rows: number } | null };
}

requestedSize is a holder cell (not a plain field) because the outer ws message handler reassigns it AFTER helpers capture the context — they need to see the latest value on every poll.

Numbers

• setupNewPtyEntry: 478 → 213 LOC
• attachStreamingBackend orchestrator: 80 LOC (sequencing only)
• file net +24 LOC (helper signatures + jsdoc)

Drive-by

• removed dead let pendingPrefillMode (declared, never read)
• lifted PrefillMode + VALID_PREFILL_MODES to module scope (helpers need them)
• added explicit return types on attachStreamingBackend + spawnPty
• extracted PtyEntry interface from inline object-literal type

Verify

• bunx tsc --noEmit -p . clean
• bun test — 1483 / 1483 pass
• zero as any introduced
• deployed locally via scripts/deploy-local.sh