feat(v2): --dag flag wires v2 DAG engine end-to-end, bump to 2.0.3

[Eth-Interchained]

Summary

• nedbd --dag (or NEDBD_DAG=1): replaces the process with the v2 Rust DAG binary via os.execv — same port/data/token env vars, zero Python overhead. v1 AOF engine untouched.
• nedbd-v2 bundled in the Python wheel: CI builds it per-platform, stages into python/nedb/nedbd-v2 before maturin — pip install nedb-engine ships both engines.
• All TODO flags closed in rust/nedb-v2/:
  • Real tombstone deletes: Db::delete() writes a tombstone node + removes the id from IdIndex so queries/list never return deleted docs
  • Real BLAKE2b Merkle head: db_seq_head() hashes all sorted id-index hashes chained with seq — tamper-evident, changes on every write
  • DEK passthrough to migrate_if_needed() so encrypted v1 AOFs convert correctly
• Fix parking_lot dep: 0.12 (correct) not 1 (non-existent on crates.io)
• rust/nedb-v2 added to Rust workspace with Cargo.lock committed for reproducible CI
• Bump all version strings to 2.0.3: pyproject.toml, init.py, package.json, rust/Cargo.toml, rust/crates/nedb-py/pyproject.toml, rust/nedb-v2/Cargo.toml — all in sync, valid for both PyPI and npm (stable, no normalization needed)

Usage after merge

pip install --upgrade nedb-engine   # 2.0.3
nedbd --dag /path/to/data           # v2 DAG engine
# or
NEDBD_DAG=1 nedbd /path/to/data

Engine impact

• v1 AOF engine: unchanged
• v2 DAG engine: tombstone deletes + BLAKE2b Merkle head + encrypted migration all wired

Tests run

• All deps fetch and resolve (Cargo.lock committed)
• Python tests unaffected (v1 path unchanged)
• CI will compile nedbd-v2 binary per platform and bundle it in the wheel

Follow-up

• Merge to master, tag v2.0.3 — CI ships PyPI 2.0.3 and npm 2.0.3 (both clean stable semver)
• Vision VPS: NEDBD_DAG=1 nedbd --data /fresh/data-dir after upgrade