bug(rvf-runtime): RvfStore::open() resets metric to L2 and zeroes last_witness_hash

[aepod]

Summary

RvfStore::create(path, options) honors options.metric (e.g. DistanceMetric::Cosine).
RvfStore::open(path) constructs:

let opts = RvfOptions {
    domain_profile,
    ..Default::default()  // metric: L2
};
// last_witness_hash: [0u8; 32]

boot() restores dimension and profile from the MANIFEST, but never restores metric. Metric is not written into the manifest. A reopen silently switches distance ranking to L2 and restarts the witness chain at zeros.

Affected versions

• Confirmed: 0.2.0 store.rs open / boot
• Still present in spirit: 0.3.0 (open still ..Default::default(), no metric on parsed manifest)

Repro

use rvf_runtime::{DistanceMetric, RvfOptions, RvfStore};

fn main() {
    let dir = tempfile::tempdir().unwrap();
    let path = dir.path().join("metric.rvf");
    {
        let opts = RvfOptions {
            dimension: 3,
            metric: DistanceMetric::Cosine,
            ..Default::default()
        };
        let mut s = RvfStore::create(&path, opts).unwrap();
        let a = [1.0f32, 0.0, 0.0];
        let b = [10.0f32, 10.0, 0.0];
        s.ingest_batch(&[&a, &b], &[1, 2], None).unwrap();
        // If metric() accessor exists (0.3+): assert_eq!(s.metric(), Cosine);
        let _ = s.last_witness_hash(); // non-zero after witness_ingest
    }
    let s2 = RvfStore::open(&path).unwrap();
    // Actual: distance path uses L2; last_witness_hash() == [0; 32]
    // Expected: Cosine preserved; witness chain tip restored from file
    let _ = s2;
}

Expected

1. Persist metric (and other durable options) in MANIFEST.
2. Restore on open/boot.
3. Restore or recompute last_witness_hash from the latest WITNESS segment so reopen continues the chain.

Optional API: open_with_options only for non-persisted overrides.

Downstream impact

WeftOS never uses Cosine at create time: we L2-normalize all vectors and always create with L2 (agenticow pattern) so reopen cannot change ranking. Witness restart on reopen is documented as a residual. Prefer a durable metric so other consumers are not foot-gunned.

Reported from WeftOS WEFT-662.