refactor(timing): extract after()/TimerHandle into SwiflowTiming with a host ManualTimers arm

[zzal]

Part-1 modularization-audit fix (finding 2).

Problem. SwiflowUI's only use of SwiflowDOM was Toast's auto-dismiss timer — a view library depending on the render backend for a 101-line utility. Worse, the timer was gated on canImport(JavaScriptKit) (true on host), so a host harness mounting ToastView walked straight into JSObject.global.setTimeout! and trapped; auto-dismiss and its WCAG pause were demo-verified only.

Change.

• New leaf target SwiflowTiming owning after()/TimerHandle, split on arch(wasm32) per the repo rule. The wasm arm keeps the JSOneshotClosure implementation verbatim (same release contract); the host arm queues into ManualTimers — a public, test-driven queue (advance(by:), reset(), pendingCount) mirroring the ManualClock pattern.
• SwiflowDOM re-exports the module (@_exported), so app code that imports SwiflowDOM (e.g. GridBoard) compiles unchanged.
• SwiflowUI now depends on SwiflowTiming instead of SwiflowDOM; Toast's #if canImport timer gates are gone.
• New ToastTimerTests (serialized suite owning the ManualTimers seam): countdown arms on mount and dismisses exactly once; hover pause re-arms the FULL duration on leave (WCAG 2.2.1); focusin/focusout pause; the countdown stays paused while either hold remains.

Verification. Whole-package swift test: 1901 tests, 351 suites, green. SwiflowUIDemo and GridBoard wasm builds pass locally (CI skips examples); GridBoard exercises the re-export path.

🤖 Generated with Claude Code

[github-actions]

📦 Bundle size

Artifact	Baseline	This PR	Δ
App.wasm	5.06 MB	5.23 MB	+3.46%
App.wasm (gzip)	1.80 MB	1.87 MB	+3.57%
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	+3.55%

✅ Within budget (≤5% growth allowed).

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