Followup: `Ownable refactor -> replace pk with sk #466`

Generalize Either<Bytes<32>, ContractAddress> helpers into Utils

Several Ownable circuits hand-roll the same Left<Bytes<32>, _> plumbing.
Lift them into Utils.compact so AccessControl / Fungible can share.

• _unsafeUncheckedTransferOwnership(left<Bytes<32>, ContractAddress>(default<Bytes<32>>)) — the verbose Either literal in renounce/zero paths.
  Refs: L191, author agreed in r3167454830 / r3167769096.
• Generalized isContractAddress (carried over from the AC PR discussion).
  Refs: L240, L138, confirmed r3202262467.
• Generalize isKeyOrAddressZero (or at least its Left side).
  Refs: L321 / L321, confirmed r3202256488.

Shared AccountId module

Ownable now carries _computeAccountId + the secret-key witness pattern.
AccessControl (and any future identity-bearing module) will want the same
primitives. Consider an AccountId module that owns only the state and
circuits and is composed into the rest.
Ref: L310.

Polish

• Tighten the header note on contract-owner authentication: until C2C
  calls land, a contract simply cannot authenticate as owner.
  Ref: r3167041751.
• Style nit: assert(_computeAccountId() == _owner.left, "Ownable: caller is not the owner"); in _assertOnlyOwner.
  Ref: r3166987733.

Start with the Ownable circuits and the existing Utils.compact helpers, then compare the AccessControl and Fungible needs described here. Determine how the shared AccountId primitives and generalized Either helpers should be composed, and verify that the listed Ownable cleanup items and shared-use cases are covered.