ADR-020: Universal ID mapping — fix the flat embedding identity problem

THE REAL PROBLEM: Every new data source produces different doc_id
prefixes in vector indexes (W-, W500K-, W5K-, CAND-). Hybrid search
had to hardcode strip_prefix for each one. New datasets broke hybrid
until someone added another prefix. This violates "any data source
without pre-defined schemas."

THE FIX: IndexMeta.id_prefix — the catalog records what prefix each
index uses. Hybrid search reads it and strips automatically. Legacy
indexes fall back to heuristic stripping. New indexes can set
id_prefix=None to use raw IDs (no prefix, no stripping needed).

This means: ingest a new dataset, embed it, hybrid search works
immediately without code changes. The system is truly source-agnostic.

Also: full ADR document at docs/ADR-020-universal-id-mapping.md
with the three options considered and rationale for the chosen approach.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/vectord/src/refresh.rs

@@ -307,6 +307,7 @@ async fn try_update_index_meta(
         chunks_per_sec: 0.0,
         bucket: "primary".to_string(),
         vector_backend: shared::types::VectorBackend::Parquet,
+        id_prefix: None,
     };
     index_registry.register(meta).await
 }