Publish bounded semantic snapshots for native accessibility #541

Parent epic: #262. Also advances nested-webview acceptance in #268. Native peer publication is owned by SceneTech/AppScene#30.

Proven gap

WebScene exposed visual scene state but no bounded, generation-stamped semantic provider that AppScene could project into NSAccessibility, UIA, or AT-SPI peers. The unchanged Code OSS workbench and nested documents therefore lacked a reusable native accessibility handoff.

Scope

• Add immutable acquire/release semantic snapshot leases.
• Publish deterministic composed-tree preorder across top and nested documents.
• Include document/frame/navigation/layout generations, stable semantic IDs, tree indices, roles, computed names/values/descriptions, states, relationships, focus, origins, and transformed bounds.
• Omit hidden, inert, detached, and non-rendered subtrees while retaining hidden IDREF name sources.
• Build on demand on the worker and prevent stale in-flight publication after navigation, low memory, or teardown.
• Keep held leases valid after publication replacement or engine destruction.

Hard bounds

• 16,384 nodes
• 256 documents
• 65,536 relationships
• 4 MiB UTF-8 arena
• 64 KiB per computed alternative

Acceptance

• Additive ABI, documentation, contracts, profile, and focused source regressions are authored.
• git diff --check passes.
• Focused execution, package testing, assistive-technology acceptance, and performance measurement run in the deferred validation lane.
• AppScene#30 consumes snapshots into native platform peers.
• Follow-up action/edit routing, incremental deltas, and live-region streams are implemented.

No build, test, benchmark, or formatter was executed under the current fast-merge instruction.