fix: validate action broker result timeout

[twilightsaurus]

Summary

Implements Slice 23: ActionBroker Timeout Validation.

Closes the deferred hardening gap where ActionBroker could be constructed directly with invalid result timeout values.

Validation

ActionBroker now requires result_timeout to be:

• finite
• strictly positive

Rejected immediately:

• zero
• negative finite values
• positive infinity
• negative infinity
• NaN

Valid finite-positive values remain unchanged.

No coercion, clamping, fallback, upper bound, dependency, or new setting was added.

Behavior preserved

No changes to:

• /action
• /action-consumer
• exact context fencing
• consumer ownership
• one-in-flight semantics
• result status vocabulary
• cancellation
• disconnect behavior
• retry/replay policy

Existing timeout behavior remains:

• timed-out action returns unknown
• detail remains consumer result timed out
• late result is rejected
• in-flight broker state is released
• subsequent actions continue normally

Production impact

Production still constructs ActionBroker() with the existing default:

5.0 seconds

Current production configuration does not expose an action-result timeout override.

Existing stale/authentication timeout validation remains unchanged.

Validation

Passed:

• focused broker/action tests
• relay suite: 226
• protocol tests: 64
• desktop tests: 152
• shared adapter tests: 31
• Mudlet tests: 32
• TinyFugue tests: 101
• Python Ruff / formatting / mypy
• TypeScript/Svelte checks
• cross-component E2E
• docs/version checks
• frontend production build
• git diff --check origin/main
• full npm run check

Independent review found no remaining High, Medium, or Low findings.

Historical audit remains unchanged.