Clarify cross-process IPC guarantees

[deverman]

Outcome

A maintainer can read the IPC spec and correctly distinguish per-process Bridge admission from unmeasured cross-process behavior, without mistaking dispatch/request.json for the active request selector.

What changed

• Document the URL argument and UUID-named file as the authoritative request-selection path.
• Scope the shared Bridge lane to service instances inside one server process.
• Clarify that per-request locks do not serialize independent request IDs.
• Record dispatch/request.json as a retained compatibility artifact, not concurrency control.
• Link the bounded cross-process measurement and decision gate in Measure cross-process Bridge admission across concurrent terminal MCP clients #216.

Why

The IPC spec said concurrent servers were serialized by the process-wide Bridge lane. That actor is shared only within one executable, so the documentation overstated the guarantee and obscured the evidence gap tracked by #216.

Validation impact

docs

Acceptance journey

A maintainer follows the documented happy path, identifies the request ID passed through the OmniFocus URL as authoritative, and sees that behavior across independent focusrelay serve processes remains unvalidated rather than guaranteed.

Validation

• swift run focusrelay-dev validate --impact docs
  • whitespace check passed
  • local Markdown links passed
• git diff --check

Closes #217.