Add optimize command for space reclamation after deletions

Implements a custom 'optimize' command (similar to SQLite FTS5) that allows
reclaiming disk space after DELETE operations:

  INSERT INTO vec_table(vec_table) VALUES ('optimize');
  VACUUM;

How it works:
- Identifies fragmented chunks from deletions
- Migrates all vectors to new, contiguous chunks
- Preserves partition keys and metadata during migration
- Deletes old fragmented chunks
- Allows VACUUM to reclaim freed disk space

Implementation details:
- Adds hidden 'table_name' column to trigger special insert commands
- vec0Update_SpecialInsert_Optimize(): Main optimization logic
  - Iterates all rows and copies to new chunks
  - Copies metadata values to new chunk positions
  - Cleans up old chunks and vector data
- vec0Update_SpecialInsert_OptimizeCopyMetadata(): Handles metadata migration

Schema improvements:
- Change PRIMARY KEY → INTEGER PRIMARY KEY in shadow tables
- Makes rowid an alias instead of separate index
- Reduces storage overhead and improves performance

Use cases:
- After bulk deletions to reclaim disk space
- Periodic maintenance to defragment vector storage
- Before backups to minimize database file size

Caveats:
- Can be slow on large tables (rebuilds all chunks)
- Should be run during maintenance windows
- Not transaction-safe for concurrent reads
- Requires VACUUM afterward to actually free space

Merged from upstream PR asg017#210 by wilbertharriman.
Fixes issue asg017#185.

Co-Authored-By: Wilbert Harriman <[email protected]>

Author: vlasky