Harden id()-keyed posterior caches with weakrefs (ProxySVAR, MaxShare, LongRunRestriction) #203

From review of #200. Three schemes now key memoisation on id(posterior) (ProxySVAR on main, MaxShare on #200, LongRunRestriction on #183). If a dead posterior's address is reused by a new object of matching shape, a stale cached result returns silently (did not reproduce in 200 forced GC attempts, but the exposure is real). Approach: store weakref.ref(posterior) as a validity token (dead ref ⇒ cache miss) in one pass across all three schemes after both branches merge. Files: src/impulso/identification.py. Related: #186 (spectral-radius dedupe).