Backfills the E4 deep-review's deferred coverage for the subview history
primitive. The keeper case is the S2 bookmark fix: a popstate that lands on a
URL-only entry (no history.state._sub) must restore the sub-view from the ?v
param via the SAME fallback the mount seed uses, not collapse to root.
Covers: seed from URL vs state._sub vs root, the no-SubviewUrl state-only
path, open() push+mirror + encode→null key-clear, and popstate restore from
state / from URL (the fix) / collapse-to-root.
9 tests, gate green.
peter
e8a1c252a9