Add an append-only ledger of entitlement set rule changes with per-pool
effect rows, a preview-and-commit rule change flow, and an automatic
drain that settles deferred recomputations. Rules gain a tier reduction
policy, resource keys declare over-limit behavior, and the materializer
now lowers limits when a rule stops applying.
Add entitlement set rule change ledger and preview flow
Add an append-only ledger of entitlement set rule changes with a
preview-and-commit operator flow. Rule writes now go through an enclosed
`core.commit_rule_change` function that files an act row and one
obligation per carrying pool, with a drain workflow settling deferred
recomputations. The preview dry-runs the materializer with a rule
overlay and renders per-pool buckets, reduction-policy disclosures, and
provider over-limit consequences. Materializing transactions take a
shared advisory rendezvous that rule changes hold exclusively, enforced
by a possession assertion. Add History and Entitlement changes surfaces,
a rule-less warning on five product-selection surfaces, and a
`tier_reduction_policy` column that gates FedWiki parking.
