Correctness fixes on top of policy-registry impl

[amiecorso]

Summary

Three atomic commits on top of impl/policy-registry from a review pass on
the reference implementation. Scope is correctness only — no style cleanup,
no tests (separate pass). Each commit is independently reviewable; build
clean throughout.

Commits

1. Pack constituent type bit in compound slot for 2-SLOAD hot path (c816cac)

The original _checkRole docstring claimed "at most 2 SLOADs for any policy"
but the implementation actually did 3 for compound: compound slot +
constituent slot (loaded only to read its type byte) + member set. Reduces
each compound constituent ID from 62 to 61 bits and uses the freed bit per
constituent to cache whitelist-vs-blacklist type at compound creation. Hot
path now reads only the compound slot + the relevant member set, matching
the docstring claim. Every B-20 transfer pays this path on every call, so
the saved SLOAD compounds at protocol scale.

2. Bound policy ID counter to 61-bit packable range (d1bba93)

The interface exposes uint64 IDs but compound slots only fit 61 bits per
constituent. _nextPolicyId now reverts with PolicyIdOverflow before
issuing an ID that would silently truncate when packed. Practically
unreachable (counter would need to reach 2^61 ≈ 2.3e18) but the alternative
was an unchecked truncation hazard hidden behind a comment. Halt-on-overflow
is preferable to silent-corruption-on-overflow.

3. Two-step admin transfer + one-way freezePolicy (91faa39)

Replaces single-step setPolicyAdmin (typo bricks a sanctions list) with
the OZ Ownable2Step shape: beginPolicyAdminTransfer /
acceptPolicyAdminTransfer / cancelPolicyAdminTransfer +
pendingPolicyAdmin view. Adds freezePolicy as a one-way switch that
permanently locks both membership and admin. The frozen flag lives in
bit 168 of the simple-policy slot (above the 160-bit admin, in
previously-unused space).

Freeze is a separate concept from admin renunciation because setting admin = address(0) would break the existence sentinel: a WHITELIST policy (type
byte = 0) with admin == address(0) would pack to literally 0,
indistinguishable from "never created."

Design choices worth a second look

• _requireConstituent returns a PolicyType placeholder (WHITELIST)
  for built-ins. Inert because the hot path short-circuits on built-in ID
  before consulting type, but a subtle coupling — if anyone refactors
  _checkRole and drops the short-circuit, the placeholder becomes a real
  bug. Documented inline.

• freezePolicy is one-way and cannot be unfrozen. Considered making it
  reversible but "frozen but unfreezable" isn't a meaningful guarantee for
  downstream integrators.

• acceptPolicyAdminTransfer preserves the frozen bit defensively.
  Currently dead code (the freeze check short-circuits earlier) but kept in
  case the semantics ever change to allow rotation while frozen.

• beginPolicyAdminTransfer(id, address(0)) silently clears the pending
  slot. A second cancellation path; could be tightened by requiring
  nonzero newAdmin and forcing callers to use cancelPolicyAdminTransfer.

• cancelPolicyAdminTransfer reverts with NoTransferPending when there's
  no pending admin. Alternative is silent no-op; went with revert because
  cancelling nothing suggests a caller mental-model bug worth surfacing.

Out of scope

• Style cleanups (named arguments at multi-arg call sites, pragma pinning).
• Tests.
• The createPolicyWithAccounts dedup observation — not a bug, sloppy on
  caller's part only.
• isAuthorized SLOAD dedup — modest cleanup, not in this PR's scope.