fix(boot): preserve reentrant root lifetimes during cleanup

[smiggleworth]

Root cleanup could erase a replacement mounted by a cleanup callback: the old cleanup continued deleting the element's registration after the new mount completed. A stale callback unsubscribe could clear the new lifetime's callbacks, and the intercepted innerHTML = '' setter could remove the replacement DOM after cleanup returned.

Retire the root registration and capture its callbacks before teardown invokes user code. Restore a reusable instance only when no replacement claimed the root; a reentrant mount supersedes an interrupted update or DOM clear. Callback unsubscription checks collection identity. Shared root initialization and default portal cleanup registration are consolidated; public signatures and the bundle limit are unchanged.

Validation:

• Six baseline regressions and three additional stale-innerHTML regressions demonstrated red before remediation. All 11 lifetime cases now pass in jsdom and native Chromium, including ordinary DOM clearing without a replacement.
• Component cleanup, ref(null), root callbacks, and strict throwing cleanup preserve replacement DOM, click listeners, registration, and resource signals. Replacement cleanup runs exactly once and aborts its own signal.
• Format, lint/typecheck, build, full unit/repository/DOM/browser suites, public types, and 21 packed-consumer tests pass on the rebased candidate. Lint retains one pre-existing unused-parameter warning.
• The SSG hydration fixture passes its unchanged 256 KiB budget.
• The component-depth benchmark passes its 10000-component regression and 1000-component mount/cleanup workload. This is a local benchmark run, not a speedup claim.