feat(storage): free-list + VACUUM to reclaim orphan pages (SQLR-6)

[joaoh82]

Summary

• Adds a SQLite-style persisted free-page list at header.freelist_head (bytes [28..32], format version v6 on demand) plus a new FreelistTrunk page type. After DROP TABLE / DROP INDEX / ALTER TABLE DROP COLUMN, the dropped object's pages move onto the freelist instead of triggering a global page-number renumber.
• Replaces save_database's linear next_free_page counter with a PageAllocator that draws from per-table preferred pools → global freelist → file extension. Unrelated tables keep their page numbers across drops, so the diff pager skips writing their byte-identical leaves.
• New VACUUM; SQL statement (executor.rs, mod.rs dispatch) compacts the file by ignoring both pools and allocating linearly from page 1, then double-checkpoints so the on-disk shrink is visible immediately. Bare VACUUM; only — modifiers rejected.

Test plan

• cargo fmt --all -- --check
• cargo clippy --workspace --exclude sqlrite-desktop --exclude sqlrite-python --exclude sqlrite-nodejs --all-targets — no new errors
• cargo test --workspace --exclude sqlrite-desktop --exclude sqlrite-python --exclude sqlrite-nodejs — 441 tests pass (9 new SQLR-6 tests + 11 unit tests for allocator/freelist)
• cargo doc --workspace --exclude sqlrite-desktop --exclude sqlrite-python --exclude sqlrite-nodejs --no-deps builds clean
• REPL smoke test: create 2 tables, populate, DROP TABLE accounts; VACUUM; → reports 2 pages reclaimed (8192 bytes), file shrinks on disk, reopen confirms remaining table's rows intact

Out of scope (follow-up tickets to be filed)

• Auto-VACUUM after every drop (the task explicitly defers this — wait for usage data).
• Cross-session freelist reconciliation beyond fs2 advisory locks.
• Bounded on_disk cache (pre-existing pager limitation, unchanged by this PR).

🤖 Generated with Claude Code