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	private_content
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 (4199652938 → 1309912954). 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_* / uniswap params renamed.
• Aztec.js / stdlib: L1ToL2Message.{publicContentHash,privateContentHash}, L2Claim.{claimPrivateContent,claimPrivateContentHash}, generateClaimPrivateContent, computePrivateContentHash; bot store schema bumped to v2.
• CLI: get-l1-to-l2-message-witness --private-content, --payment ...,claimPrivateContent=, claimPrivateContent JSON/log key.

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

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.
• Not yet verified locally (artifact rebuild was in progress when I opened this): PXE private_execution.test.ts L1→L2 cases and TXE-backed l1_to_l2_messages.nr tests — these depend on freshly compiled contract artifacts; relying on CI for them and the cross-chain e2e suites.