feat: replace ledger LMDB index with Fjall

[bmuddha]

What changed

• Replace each superblock's LMDB index with three Fjall keyspaces committed atomically at block boundaries.
• Encode ordered slot and account-span key components in big-endian form while retaining little-endian opaque span values; account history uses pubkey_prefix || execution_span keys with empty values.
• Publish ordinary blocks with buffered durability and use data synchronization for explicit sync, seal, reset, shutdown, and retention boundaries.
• Open sealed indexes lazily, exempt the two sealed predecessors of the active head from idle eviction, and close older indexes after ten idle minutes.
• Purge retained superblocks synchronously and move the remaining heed transaction helpers from nucleus into AccountsDB's index module.

Closes #62

Impact

The ledger index format is intentionally incompatible with existing LMDB ledgers and has no migration path. Blockstore and execution stream formats are unchanged. Ordinary block publication avoids per-block device synchronization; explicit strong boundaries retain data-sync durability. Fjall LZ4 compression remains disabled because the realistic fixture reduced closed-directory size by only 7.37%.

Reviewer notes

The highest-risk invariants are cross-keyspace batch publication, reverse ordering and exclusive pagination for account history, and metadata publication only after the selected file/index durability boundary. Retention removes metadata durably before synchronously purging the sealed directory.

[coderabbitai]

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[bmuddha]

superseded with #71