chore: WIP-1002 Improvements

[Kemperino]

1. Rename some functions
2. Bettter spec out the pinned storage
3. Save credential -> subsidy budget in bitmap to reduce storage requirements.

Note

Low Risk
Documentation-only changes to a draft WIP; no implementation or on-chain behavior in this PR.

Overview
This PR refines WIP-1002 (World ID subsidy accounting spec) with clearer naming, a formal pinned-storage contract, and small editorial fixes—no runtime code in the diff.

API and terminology: updateRecord → updateSubsidyClaim, with matching signal struct/tag (UpdateSubsidyClaimSignal), error SubsidyClaimDoesNotExist, and view renames: getRemainingBudget, getAvailableBudget, getAuthorizedNullifiers, hasClaimedCredential. Examples, replay-protection text, and the per-tx debit pseudocode are updated to match.

Pinned storage (main substantive addition): New ERC-7201-style section defines PinnedSubsidyStorage, PINNED_STORAGE_LOCATION, namespace derivation, fixed offsets for budget / nullifiersOf, concrete keccak slot formulas for mappings and dynamic arrays, uint128 budget encoding rules, and a requirement that Solidity and the revm extension share this layout (with golden tests recommended). Non-pinned maps (auth, authorized, claimed, etc.) are called out separately; security text now lists the full pinned commitment set.

Minor: Typos/clarity (storage layout is, debit hook references pinned derivation); architecture bullet lists updateSubsidyClaim instead of updateRecord.

^{Reviewed by Cursor Bugbot for commit 298e8a9. Configure here.}