chore(fast-inbox): delete legacy L1 inbox path (A-1386)

[spalladino]

Deletes the legacy L1 Inbox path now that propose enforces streaming-inbox consumption (AZIP-22 Fast Inbox, FI-16). Stacked on spl/a-1385-streaming-e2e; part of the Fast Inbox stack (#24784..#24790 below this one).

What is deleted

• Frontier trees: the trees mapping, forest, HEIGHT/SIZE/EMPTY_ROOT, the per-message tree insert, getRoot(), and the whole consume() flow.
• LAG / inboxLag: the LAG immutable and _lag/_height constructor args (the Inbox constructor is now (rollup, feeAsset, version, bucketRingSize)), RollupConfigInput.inboxLag and its RollupCore pass-through, and Constants.L1_TO_L2_MSG_SUBTREE_HEIGHT from the Inbox construction.
• Dead event field: the MessageSent.checkpointNumber (former inProgress) argument, which became meaningless once consume() stopped advancing it.
• Orphaned errors: Inbox__Unauthorized, Inbox__MustBuildBeforeConsume, Rollup__InvalidInHash.

What is re-homed / kept

• The compact message index now reads the current bucket's running total (totalMsgCount) instead of a parallel counter; InboxState.inProgress and getInProgress() are gone.
• The 128-bit rollingHash, InboxState.{rollingHash, totalMessagesInserted}, getState(), and getTotalMessagesInserted() are kept: the node still consumes them for message sync and L1-reorg detection. Their removal is the node cleanup's job (FI-18).
• FrontierLib is kept — it still backs the base-parity (test/Parity.t.sol) and merkle (test/merkle/Frontier.t.sol) tests, which are unrelated to the Inbox (parity-circuit territory, FI-17). The plan's "delete the file" assumption is contradicted by grep.

TS follow-through

• ethereum InboxContract: drop getLag(), drop the MessageSent.checkpointNumber decode, stop reading inProgress; InboxContractState.treeInProgress is now optional (no longer tracked on-chain).
• Stop threading inboxLag through queries.getL1ContractsConfig and the L1 deploy env. The broader AZTEC_INBOX_LAG config sweep (ethereum/src/config.ts, foundation, stdlib, ~30 e2e configs) is deferred to FI-18, which explicitly owns it.
• Archiver decode derives the message's checkpoint number from the compact index (the event no longer carries it), keeping the legacy per-checkpoint store shape untouched for FI-18.
• Removed the obsolete advanceInboxInProgress cheat code + its inbox-drift tests, and the orphaned archiver inHash-mismatch sync test (the cross-check was removed at the flip).

Gas

Deleting the frontier insert is a large sendL2Message win. Whole-test gas (forge test on InboxBuckets.t.sol), before → after:

Case	Before	After
First-ever message	175,251	116,835
First message of a new L1 block	329,460	195,555
Absorb into an existing bucket	286,673	149,968
Rollover mid-block (256 messages)	18,923,988	3,068,480

Per-call sendL2Message gas after cleanup: first-ever 99,526; first-of-new-block 53,763; existing-bucket absorb 9,273; rollover 53,801.

Testing

• forge build + forge test green: 887 passed, 0 failed. This fixes 4 pre-existing baseline failures the flip stranded (fee_portal/TokenPortal deposit tests using tree-relative indices + the old event shape).
• @aztec/ethereum builds clean; config/queries unit tests green.
• @aztec/archiver has no self-owned type errors; message_store (36), archiver-sync (59), and the decode/struct suites (58) all pass. (Pre-existing noir-protocol-circuits-types stale-artifact errors are unaffected.)
• e2e not run locally. The stability gate ("a few days of green e2e/networks before deleting the fallback") applies at merge time for this stacked line, not at PR creation.

Review follow-up (phase-2 final review)

Deleting the inbox-drift bot test left bot.test.ts's cheatCodes variable unused (lint error); a follow-up commit removes it.