refactor: per-session locking in SessionManager (audit round 3, P7)

[mizu-jun]

Implements finding P7 from the round 3 audit (docs/plans/audit-round3-2026h2.md).

Problem

The sessions map was a single Arc<Mutex<HashMap<String, Session>>>. Every IPC operation locked the whole map, so any action on one session (key input, resize) blocked every other session and client behind the same lock.

Change

Map is now Arc<Mutex<HashMap<String, Arc<Mutex<Session>>>>>.

• New SessionManager::session_arc(name) holds the map lock only to look up and clone a session's Arc, then releases it. Callers lock just that one session — the canonical by-name path.
• All ~50 IPC call sites (pane / window / session / file dispatch, ws, handler) and the internal manager methods migrated to this pattern.
• Lock order (deadlock prevention): map → session → workspace_state. No two of {session, workspace_state} are ever held together. list_workspaces, rename_workspace, delete_workspace, to_snapshot, and poll_foreground_processes were restructured to clone session Arcs under the map lock, then lock each session separately.
• pane_snapshot (sync, WASM plugin host) uses try_lock on both the map and each session, failing closed as before.

Per-session window/pane lock granularity is intentionally left as a future item.

Tests

• New concurrent_manager_ops_do_not_deadlock: drives workspace + sessions-map + snapshot paths from 8 concurrent tasks under a 10s timeout — PTY-free, so it runs in CI (unlike the session-creating tests, which spawn PTYs and are #[ignore]).
• cargo test (all crates, 26 suites), cargo clippy --all-targets -- -D warnings, cargo fmt --check all green.

Notes

The migration bulk was mechanical (same transform at every call site); reviewed for the lock-order invariant above. Real throughput improvement can't be measured headless — the win is that per-session operations no longer serialize on one global lock.

Remaining deferred audit items: P3, P6 (profiling), P5, R5/A5 (PROTOCOL bump).

[github-actions]

Coverage report

Generated by cargo llvm-cov (workspace minus nexterm-client-gpu and nexterm-i18n).