fix(lifecycle): detect parent death on Windows via PID existence probe (#982)

[alove20]

Fixes #982.

startLifecycleGuard runs on every platform (server.ts), but makeDefaultIsParentAlive (src/lifecycle.ts) only recognizes parent death two ways: the direct ppid changing (POSIX reparent-to-init), or the grandparent becoming PID 1 (a ps probe that returns NaN on Windows).

On Windows neither fires. A child is not reparented when its parent dies — process.ppid keeps returning the original (now-dead) value — so the equality check stays green and the ps grandparent probe is unavailable. Result: isParentAlive() returns true forever, the guard never calls onShutdown, and the MCP server orphans and lingers after the Claude Code session ends/crashes/reboots (pre-existing busy-wait then pinned a core — see #985).

Change: add a positive PID existence probe via process.kill(ppid, 0) (ESRCH ⇒ gone, EPERM ⇒ alive). This is the one death signal that works cross-platform, and it's strictly additive — POSIX keeps its existing faster paths. The probe is injectable (isPidAlive) for deterministic tests.

Adds a #982 test proving a stable-ppid + NaN-grandparent instance is reaped once the parent PID disappears, and threads the injected probe through the existing #311 grandparent tests. Full build + lifecycle/stdin/pi-bridge suites pass.

[alove20]

Framing note for reviewers: this extends the zombie/orphan-elimination strategy that ADR 0001 already relies on, to Windows.

ADR 0001 keeps the content store intentionally multi-writer and identifies the real cause of WAL bloat + ctx_search hangs (#560) as processes that never exit gracefully — so closeDB()'s wal_checkpoint(TRUNCATE) never fires and a lingering reader lock starves other checkpoints. On POSIX that zombie is eliminated by reparent-to-init detection (ppid change / grandparent→PID 1). On Windows neither signal exists: an orphaned child keeps its original ppid and readGrandparentPpid returns NaN, so makeDefaultIsParentAlive() returns true forever and the guard never reaps. This PR adds the one cross-platform death signal (process.kill(ppid, 0)), so the ADR's graceful-exit assumption finally holds on Windows too. No locking is introduced.