Extract LogEntry/PendingLogEntry traits into a new tlog_entry crate

[lukevalenta]

Third slice of #230. Move LogEntry/PendingLogEntry traits + helpers (and the c2sp.org/tlog-tiles default entry-bundle implementation) out of tlog_tiles into a new internal tlog_entry crate. After this, each spec crate matches its C2SP slug exactly.

Stacked on PR #233 (slice 2: tlog_checkpoint). Diff against #233 is the new content here; once #231/#233 land this PR's base will be retargeted to main.

Why a new crate (and not generic_log_worker)

The natural framing of #230's slice 3 was "move worker abstractions to generic_log_worker". On closer reading, the situation is more nuanced: the traits are a worker ↔ spec interface, with multiple consumers on each side:

• Implementors (spec-level): tlog_tiles (the default tlog-tiles entry-bundle), static_ct_api, bootstrap_mtc_api, soon ietf_mtc_api.
• Consumer (worker-side): generic_log_worker.

generic_log_worker already depends on static_ct_api / bootstrap_mtc_api, so moving the trait definitions there would create a cycle. Putting them in any single spec crate forces every other spec crate to depend on it. A small interface crate (tlog_entry, publish = false) breaks the tangle cleanly.

What moves to tlog_entry

Symbol	Old home	Notes
PendingLogEntry, LogEntry traits	tlog_tiles::entries	The worker / spec interface.
PendingLogEntryBlob	tlog_tiles::entries	DO storage shape.
LookupKey, LOOKUP_KEY_LEN	tlog_tiles::entries	Cache/dedup key.
TileIterator	tlog_tiles::entries	Helper for parsing entry-bundle tiles.
TlogTilesPendingLogEntry, TlogTilesLogEntry	tlog_tiles::entries	Default c2sp.org/tlog-tiles entry-bundle impl. Co-located with the trait it implements.

Crate has publish = false. Deps: length_prefixed, serde, sha2, tlog_checkpoint, tlog_core, tlog_tiles.

What stays in tlog_tiles

Just tile.rs — the c2sp.org/tlog-tiles HTTP wire format itself (Tile, TlogTile, PathElem, TileReader, TileHashReader, TlogTileRecorder, PreloadedTlogTileReader).

Production deps shrink to just tlog_core. length_prefixed, serde, sha2 dropped entirely; base64/url retained only as dev-deps for the CT integration test in lib.rs.

End state of #230

After slices 1–3 ship, each spec crate is aligned with the C2SP slug it implements:

• tlog_core → RFC 6962 Merkle math + draft-ietf-plants-merkle-tree-certs subtree proofs.
• tlog_checkpoint → c2sp.org/tlog-checkpoint signed-note format.
• tlog_tiles → c2sp.org/tlog-tiles HTTP wire format.
• tlog_entry → internal worker/spec interface (publish = false).

Migration

Every consumer of the moved symbols (bootstrap_mtc_api, bootstrap_mtc_worker, ct_worker, generic_log_worker, integration_tests, static_ct_api) now imports from tlog_entry directly.

bootstrap_mtc_worker and ct_worker no longer depend on tlog_tiles at all (they were only pulling it in for the trait definitions).

Stats

25 files changed, +109 / −44 (vs slice-2 head). The entries.rs move is git mv with 87% similarity (the diff is the new module doc-comment header + the tlog_tiles::PathElem import).

Pre-push checks

All four pass:

• cargo clippy --workspace --all-targets -- -Dwarnings -Dclippy::pedantic
• cargo test (33 test result blocks ok, 2 more than slice 2 from tlog_entry's relocated test_parse_tile_entry)
• cargo fmt --all --check
• cargo machete

Closes #230.