refactor!: rename L1->L2 message content/secret to public_content_hash/private_content

[benesjan]

Implements the rename agreed in governance discussion #52. The Inbox data model is unchanged; the names now describe what the fields are:

Concept	Old	Solidity / TS	Noir
hashed public payload	content	publicContentHash	public_content_hash
L2-side preimage	secret	privateContent (Fr[])	private_content ([Field; N])
its commitment	secretHash	privateContentHash	private_content_hash
hash helper	computeSecretHash / compute_secret_hash	computePrivateContentHash	compute_private_content_hash
domain separator	DOM_SEP__SECRET_HASH ("secret_hash")	DomainSeparator.PRIVATE_CONTENT_HASH	DOM_SEP__PRIVATE_CONTENT_HASH ("private_content_hash")

Breaking changes

• Protocol: the domain separator string was renamed, so the private content hash value changes. Pending L1→L2 messages whose hash was computed with the old helper cannot be consumed with the new one. TS↔Noir test vectors regenerated.
• L1: Inbox.sendL2Message(_recipient, _publicContentHash, _privateContentHash), L1ToL2Msg struct / MessageSent event fields, Inbox__PublicContentHashTooLarge / Inbox__PrivateContentHashTooLarge (new selectors), FeeJuicePortal.depositToAztecPublic(..., _privateContentHash) and its event, reference TokenPortal / UniswapPortal.
• Aztec.nr: consume_l1_to_l2_message(public_content_hash, private_content, sender, leaf_index), compute_private_content_hash, compute_l1_to_l2_message_{hash,nullifier} params, TXE send_l1_to_l2_message / send_l1_to_l2_message_from_private_content_hash. Applied to both aztec-nr and the aztec_sublib copy. FeeJuice.claim / TokenBridge.claim_* keep their secret params (used as the message's private content).
• Aztec.js / stdlib: L1ToL2Message.{publicContentHash,privateContentHash}, computePrivateContentHash(privateContent: Fr[]) (mirrors Noir; computeSecretHash took a single field), computeFeeJuiceMessageNullifier / getNonNullifiedL1ToL2MessageWitness take privateContent: Fr[]. generateClaimSecret removed — use Fr.random() + computePrivateContentHash. L2Claim.claimSecret / claimSecretHash keep their names; bot store schema bumped.
• CLI: get-l1-to-l2-message-witness --private-content. claimSecret= payment arg and generate-secret-and-hash unchanged (hash values differ).

Migration notes added. Out of scope: L2→L1 messages (L2ToL1Msg.content etc.), TransparentNote.secret_hash (app-level; only its compute_private_content_hash call updated), versioned docs.

Labs submodule

The labs components (yarn-project, aztec-nr, noir-contracts, docs) are built from the labs/ submodule, so the labs side of the rename is carried as labs-patches/0004-refactor-rename-L1-L2-message-content-secret-to-publ.patch (the same diff as the in-tree copies, minus the already-removed generate-secret-and-hash CLI command). It needs to be upstreamed to aztec-node (labs-patches/bootstrap.sh upstream 4).

Testing

• forge test for Inbox, buckets, fee portal, portals, rollup suites: green.
• nargo test for protocol constants_tests (derives the new separator) and aztec-nr hash vector: green.
• yarn build in yarn-project: clean. Unit tests in stdlib (hash/messaging), bot, txe: green.
• Relying on CI for PXE private_execution.test.ts L1→L2 cases, TXE-backed l1_to_l2_messages.nr tests and the cross-chain e2e suites.