feat(ipc): add opt-in admission and authorization

[rrrodzilla]

IPC clients currently share the same listener without an application admission or authorization boundary. This adds opt-in policies that bind server-established identity to a connection and expose it to typed handlers. Existing listener entry points retain their behavior.

Policies admit connections asynchronously and authorize requests, discovery, exact and pattern subscriptions, and each outgoing notification. Recipient memoization remains separate from authorization. Revocation removes routes, cancels transport activity, and skips queued IPC messages that have not begun dispatch.

Emergent can use these hooks to map live child PIDs to primitives and enforce declared capabilities. That application policy remains in Emergent.

Closes #20. Supports Govcraft/emergent#24.

Release: 9.4.0 across all four workspace crates.

Validation:

• Clippy: workspace/all targets, all features and no default features, zero warnings.
• Nextest: 686 all-feature tests and 466 no-default-feature tests passed (one pre-existing skipped test per suite).
• JSON-only IPC without default features: all seven security integration tests passed.
• Documentation examples: eight passed.
• Semver: all 196 compatibility checks passed against 9.3.0.
• Documentation build and all 2,140 links passed.
• Linux and Windows PR CI required before merge.