Bump Cargo version to 0.4.0

Releasing as v0.4.0 — substantial work since v0.3.0:
- Phase D operability completed: explain / verify endpoints,
  rollup→raw provenance closing spec §19.10, Parquet export module,
  admin CLI with 5 subcommands
- Operational safety: exclusive DB lock (flock on db_root/LOCK),
  sort-on-flush in canonical billing order, RLE encoding for `kind`
- 23 new tests (91 → 114)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: pbudzik

Cargo.lock

@@ -1,6 +1,6 @@
 [package]
 name = "usagedb"
-version = "0.3.0"
+version = "0.4.0"
 edition = "2024"
 license = "Apache-2.0"
 description = "Embedded append-only usage database for AI billing workloads"