Refactor cross-cutting stable-memory and VFS plumbing

[humandebri]

Summary

• Reorganize the stable-memory, metadata, and memory-manager layers behind a clearer module split
• Update SQLite VFS, stable-blob, and connection code to reflect the new structure
• Refresh tests and profiling/read-metrics integration to cover the refactor
• Refresh public API and compatibility lock snapshots for the new layout

Testing

• cargo fmt --all --check
• git diff --check
• rg -nUP '[\x{061C}\x{200E}\x{200F}\x{202A}-\x{202E}\x{2066}-\x{2069}]' . (no matches)
• cargo check --no-default-features --features sqlite-bundled
• cargo check --features canister-api
• cargo check --features bench-profile
• cargo check --target wasm32-unknown-unknown --no-default-features --features sqlite-precompiled
• cargo check --target wasm32-unknown-unknown --no-default-features --features sqlite-precompiled,canister-api
• cargo test
• VERUS_REQUIRED=1 bash scripts/sqlite-critical-check.sh

CI failure notes:

• The failing VERUS_REQUIRED=1 bash scripts/sqlite-critical-check.sh run first stopped because docs/PUBLIC_API_2_0.snapshot was stale after moving src/db/connection.rs to src/db/connection/mod.rs.
• After fixing that, the same check exposed stale compatibility lockfiles caused by making candid / ic-cdk optional; those lock snapshots are now refreshed.

Invalid matrix commands for the current feature design:

• cargo check --no-default-features intentionally fails because no SQLite backend is selected.
• cargo check --no-default-features --features sqlite-precompiled is only valid for wasm32-unknown-unknown; native targets intentionally fail.
• cargo test --all-features intentionally enables both sqlite-bundled and sqlite-precompiled, which are mutually exclusive.