matrixd: multi-corpus retrieve+merge — SPEC §3.4 component 2 of 5

Lands the matrix indexer's first piece per docs/SPEC.md §3.4:
multi-corpus retrieve+merge with corpus attribution per result.
Future components (relevance filter, downgrade gate, learning-loop
integration) layer on top of this surface.

Architecture:
  - internal/matrix/retrieve.go — Retriever takes (query, corpora,
    k, per_corpus_k), parallel-fans across vectord indexes, merges
    by distance ascending, preserves corpus origin per hit
  - cmd/matrixd — HTTP service on :3217, fronts /v1/matrix/*
  - gateway proxy + [matrixd] config + lakehouse.toml entry
  - Either query_text (matrix calls embedd) or query_vector
    (caller pre-embedded) — vector takes precedence if both set

Error policy: fail-loud on any corpus error. Silent partial returns
would lie about coverage, defeating the matrix's whole purpose.
Bubbles vectord errors as 502 (upstream), validation as 400.

Smoke (scripts/matrix_smoke.sh, 6 assertions PASS first try):
  - /matrix/corpora lists indexes
  - Multi-corpus search returns hits from BOTH corpora
  - Top hit is the globally-closest across all corpora
    (b-near beats a-near at distance 0.05 vs 0.1 — proves merge)
  - Metadata round-trips through the merge
  - Distances ascending in result list
  - Negative paths: empty corpora → 400, missing corpus → 502,
    no query → 400

12-smoke regression sweep all green (D1-D6, G1, G1P, G2,
storaged_cap, pathway, matrix).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

lakehouse.toml

@@ -13,6 +13,7 @@ queryd_url   = "http://127.0.0.1:3214"
 vectord_url  = "http://127.0.0.1:3215"
 embedd_url   = "http://127.0.0.1:3216"
 pathwayd_url = "http://127.0.0.1:3217"
+matrixd_url  = "http://127.0.0.1:3218"
 
 [storaged]
 bind = "127.0.0.1:3211"
@@ -54,6 +55,14 @@ bind = "127.0.0.1:3217"
 # /var/lib/lakehouse/pathway/state.jsonl so traces survive restart.
 persist_path = ""
 
+[matrixd]
+bind = "127.0.0.1:3218"
+# matrixd calls embedd (query-text → vector) and vectord (per-corpus
+# search) directly. Localhost defaults; in distributed deployments
+# these point at the gateway's upstream addresses.
+embedd_url  = "http://127.0.0.1:3216"
+vectord_url = "http://127.0.0.1:3215"
+
 [s3]
 endpoint        = "http://localhost:9000"
 region          = "us-east-1"