Issue #3615840: Let the map fetch what it needs, so the picker page carries nothing per-visitor

The picker page now carries nothing that differs between two visitors, so its session context and max-age of zero are gone and it is shareable. The map assembles from the drawing, the shared availability document, and its own places resolved from the stamp in the cookie plus the copy this browser kept: matching digests mean Drupal is asked nothing at all.

Corrections moved client-side, since state() is no longer read by any page and a shared document cannot carry them. Two things surfaced under test: a refusal does not move the digest, so the map records it in its own kept copy; and corrections must outlive an empty basket, because a booker refused a seat holds nothing at all.

venue_mine finally has a route; it was merged unreachable.

Local: 17 browser tests, 23 kernel classes, VenueMapFetchTest, PlaceBookingFormTest and PlaceMapPerformanceTest all green. The page is one query lighter; two cache writes and one tag lookup moved into the availability request. ScriptBytes raised deliberately, with the reasoning in the test.