mirror_worker: parse and verify add-entries (C4a)

[lukevalenta]

Summary

Implements the parse-and-verify portion of c2sp.org/tlog-mirror#add-entries. On a successful request the handler returns 200 with an empty body. Persistence, tile recomputation, committed-checkpoint advancement, and mirror cosignature emission land in C4b/C4c.

Towards #186 (tlog-mirror support); see the C1–C5 plan comment for how this fits into the broader mirror work. Stacked on #238; base will retarget to main as predecessors merge.

What the handler does

1. Slurp the request body, capped at MAX_ADD_ENTRIES_BODY_SIZE = 10 MiB. A knob, not a spec value, sized to fit comfortable batches without letting a hostile client push the parser into base64-decoding tens of megabytes of garbage.
2. Parse the AddEntriesRequestHeader via tlog_mirror::wire. Malformed headers, oversized bodies, or trailing bytes after the last package surface as 400.
3. Origin lookup: 404 if log_origin isn't configured.
4. Snapshot per-origin DO state via the C2 POST /get-state RPC.
5. Resolve target pending checkpoint:
   • upload_end == snapshot.pending.size: use the current pending.
   • Otherwise: open the request ticket via TicketMacer::open, re-parse the plaintext as a signed Note, re-verify against the trusted log keys, and adopt the embedded checkpoint iff its origin and size match. This implements the spec's "store the signed checkpoint in the ticket" recommendation.
   • Neither matches: 409 with text/x.tlog.mirror-info body carrying the current (pending.size, committed.size, fresh_ticket_for_current_pending).
6. Validate upload_start: <= committed.size and (C4a constraint) % 256 == 0. Non-aligned upload_start requires reading previously-committed leaves from R2 to verify the first package's subtree, which lands in C4b. C4a surfaces non-aligned starts as 409 so the client can re-fetch and realign.
7. Per-package verification via package_ranges. For each:
   • Parse wire bytes via EntryPackage::read_from.
   • Replay leaves through tlog_core::stored_hashes_for_record_hash into a sparse HashMap<u64, Hash> store. Because the package is aligned, no pre-package leaves are referenced during replay.
   • Reconstruct the package's subtree hash via Subtree::hash.
   • Verify the subtree consistency proof against the target pending checkpoint via tlog_core::verify_subtree_consistency_proof. 422 on failure.

Implementation notes

• MapReader: a sparse HashMap<u64, Hash>-backed HashReader. The store is built incrementally as we replay leaves, with a fresh borrow per iteration so the borrow checker is satisfied without unsafe.
• Indexes via tlog_core::stored_hash_index(0, n) directly — already public on the spec crate.
• mirror_info_409 seals a fresh ticket from the current pending signed-note bytes whenever it returns a 409. If MIRROR_TICKET_KEY is missing or malformed, we still return 409 but with an empty ticket and log an error so an operator notices.
• The handler trusts the streaming-friendly shape of EntryPackage::read_from. Workers' req.bytes().await doesn't actually stream, but the per-package iteration model means future Workers streaming support won't require a rewrite.

What this PR does NOT do

• Persist entries to R2 (C4b).
• Recompute tiles from received entries (C4b).
• Advance the committed checkpoint via the C2 /commit RPC (C4c).
• Emit mirror cosignature lines in the response body (C4c).
• Lift the upload_start % 256 == 0 constraint (requires C4b's R2 reads).
• Add an integration test for add-entries end-to-end. C4a's success-is-empty-body shape doesn't observably differ from a stub against wrangler dev; the meaningful integration test lands with C4c.

Stats

• 3 files changed, +475 / −5.
• 1 new file: crates/mirror_worker/src/add_entries.rs (467 LOC).
• 1 new route on the frontend, 1 new module declaration.
• 18 unit tests in mirror_worker (unchanged from mirror_worker: load cosigner + ticket keys, /metadata, extended state DO #238; verification logic exercised end-to-end in C4b/C4c integration tests rather than via narrow unit tests over Merkle math plumbing).

Verification

All five pre-push checks pass locally on the head commit:

cargo clippy --workspace --all-targets -- -Dwarnings -Dclippy::pedantic
cargo test
cargo fmt --all --check
cargo machete
cargo check -p mirror_worker --target wasm32-unknown-unknown