fix: evict mountedRoots on unmount; ManualTimers tests; PersistentStore host warn

[zzal]

Audit backlog B13 (Part-2/Part-6 Lows) — the hygiene items with behavior.

• mountedRoots had no evict path. The driver's selector→root-node map was populated on mount/replaceMount but never cleaned: a root unmounted (Swiflow.unmount(into:)) and never remounted pinned its detached node forever, and the node stayed attached to the DOM. Fix: a new unmount(selector) driver op detaches the root and drops the entry, called from Renderer.teardown(); HMR teardown clears the map alongside nodes/listeners. Crucially, mountedRoots deliberately survives a bare destroyNode (the resync-batch ordering emits the old root's destroyNode before replaceMount, which needs the entry to detach the old node) — so the evict is an explicit unmount signal, not a destroyNode side effect. Two js-driver tests pin both halves.
• SwiflowTiming had no direct tests — ManualTimers was covered only through ToastTests. New SwiflowTimingTests owns the process-global seam (.serialized) and pins schedule/advance/cancel/reset, the isolated deinit auto-cancel, due-order firing, in-body reschedule, and the 1e-9 FP-residue tolerance.
• PersistentStore's host stub no-ops silently — the first host save now emits a one-time DEBUG swiflowWarn, so a test asserting persistence against the default registry (instead of injecting MemoryStorage) gets a signal instead of a silent pass/fail.

Deliberately deferred (documented in the commit): the @State/@Persisted didSet-prologue extraction (assertMacroExpansion pins byte-exact output; SwiftSyntax multiline interpolation flattens the nesting — not worth churning ~10 goldens for a stable 6-line block) and the callback @MainActor unification (a source-breaking public-API change that belongs in an intentional breaking batch, and is invisible on the wasm target).

Verification: js-driver 105 tests, 0 fail. Whole-package swift test: 1916 green. EmbeddedDriver.swift + 22 example copies regenerated.

🤖 Generated with Claude Code

[github-actions]

📦 Bundle size

Artifact	Baseline	This PR	Δ
App.wasm	5.06 MB	5.23 MB	+3.45%
App.wasm (gzip)	1.80 MB	1.87 MB	+3.56%
JS runtime	54.5 KB	54.5 KB	+0.00%
JS runtime (gzip)	11.3 KB	11.3 KB	+0.00%
Total (gzip)	1.81 MB	1.88 MB	+3.54%

✅ Within budget (≤5% growth allowed).

_{Baseline: Swift 6.3, WASM SDK 6.3-RELEASE, measured 2026-06-18.}