fix(renderer): guard the root mount path against fragment/multi-root bodies

[zzal]

Summary

Closes Part I Wave 3 #5 of the arch+DX audit. Fragment-rooted component bodies are only DEBUG-discouraged (the bare-fragment diagnostic in Diff.swift), but the root mount path used single-valued MountNode.domHandle — whose own doc warns it does not descend through fragments and is "only valid for single-rooted nodes." For a fragment root it returned the structural handle "the DOM never sees," feeding a bogus handle to mount/replaceMount in RELEASE (where the DEBUG diagnostic is compiled out) — a silently broken mount.

Fix

MountNode.singleRootDOMHandle: descends through fragments (via collectDOMRoots) and traps in all builds (precondition — -Osize release keeps it) with actionable guidance if the node resolves to anything but exactly one DOM root. Wired into the three root-path sites in Renderer — the first-mount mount, the identity-swap replaceMount, and resyncFullRemount's replaceMount — replacing the bare domHandle.

• The DEBUG bare-fragment diagnostic stays — it's the earlier, more precise dev-time warning and covers nested fragments too; this guard is the release safety net it was missing.
• Nested fragment bodies are unaffected — they attach all their DOM roots to a parent via collectDOMRoots and never hit the root path.
• Chose enforce single-root over make the root path multi-root-aware: the latter needs driver-level "attach a list" changes and reverses the framework's current "wrap in a single element" stance.

Test plan

• 3 new host tests: the single-element root resolves correctly (agrees with domHandle), plus two exit-tests (#expect(processExitsWith: .failure)) proving a multi-root fragment and an empty root each trap — un-.disabled (unlike the DEBUG-only diagnostic tests) because the guard fires in every build.
• Full Swift monorepo suite 1370/275 (+3).
• HelloWorld wasm build + link clean.
• Counter Playwright suite 11/11 locally — the guarded first-mount + resync still mount HelloWorld's single-div root normally.

🤖 Generated with Claude Code

[github-actions]

📦 Bundle size

Artifact	Baseline	This PR	Δ
App.wasm	5.06 MB	5.15 MB	+1.91%
App.wasm (gzip)	1.80 MB	1.84 MB	+2.20%
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	1.85 MB	+2.19%

✅ Within budget (≤5% growth allowed).

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