docs: record what the e5-small swap bought (V-371)

The swap itself already landed: deploy loads
models/embedder/multilingual-e5-small/model_quantized.onnx, and
onnxembedder.go grew EmbedQuery/EmbedPassage with the query:/passage:
prefixes the model was trained with. What was missing is the half of #371
that says "re-run make eval-recall and compare against the recorded numbers",
so nothing in the repo says whether it worked.

It worked, on every axis at once. recall@1 60.0% → 70.4%, recall@3 80.0% →
85.2%, answered after the gate 48.0% → 63.0%, false recall 1/5 → 0/5, and
latency p50 59ms → 23ms because the quantized file is 118MB against the 470MB
fp32 one the old config loaded. The guitar-chords note no longer beats the
docker-logs note.

One premise of the task did not come true and the new doc says so. #371
expected a better retriever to separate the score distributions and make
query_min_score tunable. It did not: right-first top-1 runs 0.791-0.890 and
must-stay-silent runs 0.795-0.835, still overlapping, just higher and
tighter. The margin separates them instead — 0.024 median against 0.002 — and
0.008 is the knee where all five silent cases are silenced at no cost. The
score gate is close to inert now; the margin is the live dial. Neither is
changed here, since #412 is where a sweep belongs.

docs/evals/2026-08-04-recall-e5-small.md is the dated measurement.
rearchitecture.md's "upgrade MiniLM → bge-m3 later" is now done and says so,
CLAUDE.md names the retriever and the prefix rule where it already promises
the embedder never leaves homesrv, and the Makefile comment points at this
eval instead of the one that asked for the swap.

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

CLAUDE.md

@@ -32,7 +32,11 @@ See `docs/rearchitecture.md` for the target architecture, `docs/design.md` for t
 GPU and the workstation has 16GB of VRAM. So the resident model, STT and TTS become preferred
 remotes with a floor on homesrv. The workstation is never assumed up. Fall back silently when
 it would only do the job better. Name the gap when the 1.7B cannot do it at all. The embedder
-stays on homesrv permanently, because it backs that floor. Read `docs/offload.md` before
+stays on homesrv permanently, because it backs that floor. It is multilingual-e5-small,
+quantized and asymmetric — `EmbedQuery` and `EmbedPassage` apply the `query:`/`passage:`
+prefixes it was trained with, and calling plain `Embed` on a note is a bug. It replaced
+MiniLM and bought ten points of recall@1 and 2.5× the speed; see
+`docs/evals/2026-08-04-recall-e5-small.md`. Read `docs/offload.md` before
 touching a daemon seam or adding a model caller. Vikunja #483 is the umbrella, #484 to #487
 are the work.