[Reactivity] ADR + bounded .gwdk subscribe surface + IR/validation

[cssbruno]

Define the ADR-gated .gwdk subscribe surface and IR/validation path for realtime reactivity.

Scope:

• Decide whether subscribe syntax extends bounded client {} or introduces a separate source surface.
• Lower subscriptions into typed IR.
• Validate referenced presentation events against the contract registry.
• Emit diagnostics for unknown or unbounded subscriptions.

Acceptance criteria:

• ADR records the source contract.
• IR models the supported subscription surface.
• Unknown event references produce diagnostics with source spans.