feat(l1): add overlay + read cascade for deep reorgs

Builds on the journal added in #6686. Introduces:

- Overlay struct: in-memory aggregated reverse-diff bridging on-disk state
  at the cache edge D to the virtual state at a deep-reorg pivot T-1.
  Built by walking STATE_HISTORY[D..=T] descending, with older-entry-wins
  semantics so the post-walk value at every key is the pre-image at T-1.
- OverlayCf enum + classify_by_key_length, reusing PR 1's classify_trie_key.
- TrieLayerCache: optional overlay field plus set/clear/lookup accessors.
- TrieWrapper::get cascade now reads: layer cache -> overlay -> disk.

No orchestration wired yet; PR 3 will install/clear overlays via
apply_fork_choice_with_deep_reorg. 14 unit tests covering descending-range
load, older-wins, bloom miss, hash mismatch, missing entry, swapped-args
rejection, CF classification, cascade short-circuit, and edge cases.

Author: edg-l