fix: release initial superblock after rotation

[bmuddha]

What changed

• Move the initial active superblock handle into SuperblockWriter after creating the Fjall index writer.
• Allow rotation to release the appender's last reference to the previous superblock.

Closes #73

Impact

Truncated initial-superblock files, metadata mappings, and Fjall keyspace references can be reclaimed without restarting Engine. There are no API, storage-format, or compatibility changes.

Reviewer notes

IndexWriter retains its own Fjall keyspace handle and does not require the enclosing Arc<Superblock>. SuperblockWriter remains the sole appender-owned superblock handle and releases it when rotation replaces the writer.

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between 63f155b and b19526f.

📒 Files selected for processing (1)

• ledger/src/appender.rs

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