Route semantic SET_VALUE for native numeric controls #762

Parent: #262. Consumer: SceneTech/AppScene#285. Typed provider: #760 / PR #761.

Proven gap

The v2 semantic companion now publishes typed current/minimum/maximum/increment values, and AppScene PR #288 exports AT-SPI Value. Native input[type=number] and input[type=range] peers still advertise only increment/decrement, so AT-SPI CurrentValue must remain read-only even though WebScene already has the generation-bound SET_VALUE action route.

Required implementation

• Advertise semantic SET_VALUE for enabled, writable native number/range controls.
• In the worker action dispatcher, accept only finite numeric payloads, run the existing native-control sanitization/range rules, update live form state, recascade value selectors, and dispatch ordered input/change events.
• Preserve disabled/read-only/stale-generation rejection and do not advertise writes for ARIA-only custom roles without an authoritative mutation contract.
• Carry the capability through snapshots/deltas so AppScene can expose writable CurrentValue without inference.
• Add native/action/delta/Unicode-independent malformed, bounds, step, lifecycle, queue, and performance contracts.

No platform adapter may synthesize repeated increment/decrement calls to emulate an absolute set.