ConversationRouter.route() violates command-query separation (mutates lastActivity)

[Anarchid]

Found during review of #41. Design/hygiene follow-up.

Problem

ConversationRouter.route() is documented as a query ("Decide where an incoming message goes") but mutates TTL state.

src/mcpl/conversation-router.ts:157:

/** ... Pure read except for refreshing lastActivity ... */
route(facts) {
  ...
  existing.lastActivity = now;   // command hiding in a query

Impact

Command-query separation violation: you cannot preview a routing decision without resetting the idle clock. The test 'expired() respects lastActivity refresh from route()' is load-bearing on the side effect. A future diagnostic caller of route() would silently keep dying engagements alive.

Suggested fix

Split the side effect out — route() decides; an explicit touch(channelId) (or a refresh on the framework's delivery path) advances lastActivity.