perf(agent): store embeddings once, not twice

MemoryCache held every embedding in two places: a `Vec<Vec<f32>>` and a
flattened copy for the batched kernels, kept in lock-step on every push,
update and compaction. A store loaded from disk therefore carried the corpus
twice, plus one heap allocation per entry.

A new `cache::Embeddings` owns just the flat `[N x dim]` buffer and indexes
into it, so `embeddings[i]` still reads as a `&[f32]` row. The batch kernels
take a `VectorSet` (implemented for both `Embeddings` and `Vec<Vec<f32>>`)
instead of `&[Vec<f32>]`, so their callers and tests are unchanged. Loading no
longer unflattens what it just read.

100k 384-dim entries, reopened from disk: 505 -> 357 MiB, 3.44x -> 2.43x the
raw vectors. Recall (1.0000 at ef=64) and query latency are unchanged.

Rows are now always exactly `dim` long, shorter ones zero-padded. The old
representation allowed ragged rows, which silently misaligned the flattened
copy — every row after a wrong-length embedding — and `update` carried a
comment about falling back to a rebuild to avoid exactly that. It is now
unrepresentable. A record saved without an embedding holds a zero row and is
told apart by its norm, which is what `total_embeddings` now counts.

Measured with a counting allocator rather than RSS: freeing a structure
returns its pages to the allocator's pool, not the OS, so an RSS reading from
inside the process showed the two representations as identical.

Breaking: MemoryCache::embeddings changes type, embeddings_flat is replaced by
flat_embeddings(), rebuild_flat() is a deprecated no-op.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

CHANGELOG.md

@@ -2,6 +2,24 @@
 
 ## Unreleased
 
+### Memory
+- `clawhdf5-agent`: **a loaded store holds ~30% less memory** (100k 384-dim
+  entries: 505 -> 357 MiB, 3.44x -> 2.43x the raw vectors). The cache kept
+  every embedding twice — a `Vec<Vec<f32>>` and a flattened copy for the
+  batched kernels, maintained in lock-step — so it now stores only the flat
+  buffer and indexes into it. Recall and query latency are unchanged.
+  **Breaking:** `MemoryCache::embeddings` is a `cache::Embeddings` rather than
+  a `Vec<Vec<f32>>` (indexing still yields a `&[f32]` row); `embeddings_flat`
+  is gone, replaced by `flat_embeddings()`; `rebuild_flat()` is a deprecated
+  no-op. Rows are now always exactly `dim` long — shorter ones are
+  zero-padded — which makes the ragged-row case that used to silently
+  misalign the flattened copy unrepresentable.
+- `clawhdf5-bench`: `search_harness --footprint` reports live heap use per
+  stage, measured with a counting allocator (RSS cannot see a structure freed
+  into the allocator's own pool).
+
+## Unreleased
+
 ### Retrieval quality
 - `clawhdf5-agent`: **re-ranking discarded the retrieval score.**
   `reranker::rerank` built its combined score from temporal decay, source