docs: clarify sparse-not-wired and cite-boost tie-breaker (audit C-14, U-2)

- C-14: embed.py header now states that only the dense path is wired (search
  'hybrid' is dense + Postgres FTS); encode_sparse/encode are reserved for a
  future sparse-retrieval layer, not yet consumed.
- U-2: --cite-boost help clarifies it re-ranks within the top results (a
  tie-breaker), not a hard re-ranking — matching the small alpha effect.

Accepted (documented heuristics, no change): R-3 conflict detection is a
keyword-only signal labelled as such; R-12 classify_section is mostly 'body'
because chunks are word-windows that rarely start at a section header.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

codex/embed.py

@@ -1,4 +1,4 @@
-"""Hybrid dense + sparse embeddings via BGE-M3 (ADR-0002).
+"""Dense (+ optional sparse) embeddings via BGE-M3 (ADR-0002).
 
 Uses :class:`FlagEmbedding.BGEM3FlagModel` for encoding because the
 ``return_dense`` / ``return_sparse`` kwargs are part of the FlagEmbedding
@@ -7,6 +7,11 @@ vanilla ``SentenceTransformer`` load of ``BAAI/bge-m3`` (same weights,
 same model); sparse output is a list of ``{token_id: weight}`` dicts per
 text.
 
+Status: only the **dense** path is wired into ingest/search today; the search
+"hybrid" is dense + Postgres FTS. :meth:`Embedder.encode_sparse` / :meth:`encode`
+are provided for a future sparse-retrieval layer but are not yet consumed by the
+pipeline (audit C-14).
+
 Notes for callers
 -----------------
 * Empty input is handled explicitly — no model call is issued.