Document the registry membership batch limit revert so the Solidity
interface matches the Rust PolicyRegistry precompile.

Co-authored-by: Cursor <cursoragent@cursor.com>

Per review feedback: with the error declared on the interface, the
mock should mirror the Rust precompile's behavior so the same
interface-level tests pass in fork-test mode against the real
PolicyRegistry. base/base#2876 sets the limit at 64 accounts per call.

- MockPolicyRegistry: add `MAX_BATCH_SIZE = 64` (internal) and a
  pre-mutation length check in `_batchSetMembers` that reverts with
  `BatchSizeTooLarge(MAX_BATCH_SIZE)`. The check sits at the shared
  funnel so all three entry points (`createPolicyWithAccounts`,
  `updateAllowlist`, `updateBlocklist`) inherit it without
  duplication.
- PolicyRegistryTest: expose `MAX_BATCH_SIZE` as a compile-time
  constant on the test base (kept as a literal, not pulled from the
  mock, so fork tests against the real precompile share it) and add
  `_makeAccounts(n)` for batch-bound tests that need to bypass the
  existing 0..5 `_boundAccounts` cap.
- Tests: per affected entrypoint, add one revert test (fuzz over a
  size above the limit) and one inclusive-boundary success test
  (batch size exactly == MAX_BATCH_SIZE).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>