emrg: GUI multi-session P6 finalize — cancel-before-close + cap-20 prompt + projectPath validation

[argszero]

P6 finalize of the GUI multi-session rant (2026-08-10T15:07:19) — boundary hardening after P5 (#641/#642 merged).

1. Cancel in-flight stream before closing a busy connection (boundary: "关闭在忙连接先 cancel 再 close")

• ConnManager.close(sid) now detects conn.ownStream && conn.ws and sends cancel before disconnecting (fire-and-forget, swallow disconnect errors — sync close semantics preserved). Prevents dirty state from mid-stream disconnects.

2. Over-limit (20) prompt — don't auto-close (acceptance: "上限 20：超限提示不自动关")

• switchSession rejects with "too many open sessions (20) — close some first" when explicitly opening a NEW session at the cap (openSessions.size >= DEFAULT_CAP && !openSessions.has(sid)); already-open sids are reused without blocking; creation paths (sendMessage auto-open, newSession) are never blocked.
• Renderer detects the error and shows localized app.tooManyOpenSessions (zh/en) instead of leaking the raw English error.

3. projectPath validation (boundary: "projectPath 校验")

• switchSession / newSession IPC validate projectPath is a non-empty string when provided.

+3 tests: conn-manager 17→19 (busy close sends cancel / idle close doesn't), renderer.smoke 55→56 (localized over-limit message). GUI 172→175, 680 pytest green, doc counts synced.

[argszero]

✅ LGTM — cycle 20260810-184049. P6 finalize: cancel-before-close in ConnManager.close (busy streams stopped cleanly), cap-20 explicit-open guard with localized prompt (creation paths unblocked), projectPath validation. GUI 175/175, pytest 680, doc counts synced.

[pm25coder]

Tested P6 finalize end-to-end on the branch (base edf3d81, single commit 03d706e):

[argszero]

✅ LGTM — cycle 20260810-185020. Re-verified on head 03d706e: GUI 175/175, pytest 680, CI green. P6 boundary hardening correct (cancel-before-close, cap-20 prompt with creation paths unblocked, projectPath validation).

[argszero]

✅ LGTM — cycle 20260810-184923. Verified locally: GUI npm test 175/175 (conn-manager busy-close cancel / idle-close no-cancel, renderer over-limit localized prompt), pytest 680. Symbols verified: DEFAULT_CAP exported from gui-state.js, daemon supports 'cancel' (daemon.py:515), ownStream tracked on DaemonConnection (daemon_client.js:99). Doc counts synced (43+19+22+56+15+7+3+3+7=175). CI 31380737948 SUCCESS.