Use collision-free interned group keys in DAG execution

[zzylol]

Prometheus and OTLP ingress previously flattened GROUP BY values with ;. Distinct groups such as (x="a;b", y="c") and (x="a", y="b;c") therefore shared worker state, and downstream code had to reconstruct labels by splitting an ambiguous string.

This change introduces one typed GroupKey for router-to-worker execution. Its versioned, length-prefixed encoding preserves label names, values, and projection order. A bounded interner lets shared DAG consumers reuse the same immutable key and avoids rebuilding name/value allocations for repeated series. Remote-write series population keys now use the same representation, while persisted series IDs and SketchDB keys remain unchanged and require no migration.

The worker emits label values and population labels directly from the typed key. Tests cover delimiter collisions, different label names, projection order, missing values, interner reuse, and ingress routing. A Criterion case measures 100,000 high-cardinality, delimiter-rich two-label projections.

Validation:

• cargo test -p data_plane --no-run
• cargo test -p data_plane group_key --lib (4 passed)
• cargo bench -p data_plane --bench sketch_db -- group_key_projection --sample-size 10 --measurement-time 1 --warm-up-time 1
  • high-cardinality projection: median 108.48 ms / 100,000 keys (921.85k keys/s)
  • repeated shared projection: median 109.51 ms / 100,000 keys (913.13k keys/s)
• Full library run: 1,067 passed; three pre-existing stack/environment failures remain (pane-origin HTTP assertion, missing-handle HTTP status, and a fixed-port AddrInUse).