feat: Implement M2.1 Embeddings client (768-dim batching @32)

M2.1 Complete: TEI embeddings via api.riotpiao.com gateway

Implementation (crates/mem-llm/src/embeddings.rs):
- EmbeddingsClient::embed(texts) batches at ≤32 per request
- Preserves input order across batch boundaries
- Asserts 768-dim vectors, errors loudly with model name on mismatch
- Sends apikey header (future-proofing for auth plugin enablement)
- 30s timeout, retry on 5xx via reqwest Client
- Constants: EMBEDDINGS_DIM=768, BATCH_SIZE=32 (single source for schema migration)

Tests (tests/it_embeddings.rs): 8 tests
1. a1_batches_at_32 — 100 inputs → 4 requests (32+32+32+4)
2. a2_order_preserved — identifiable vectors, cross-batch order assertion
3. a3_dimension_asserted — 512-dim response → error naming model & dimensions
4. a4_apikey_sent — header present even when route doesn't require auth
5. a5_live_dims — #[ignore] live gateway test (768-dim confirmation)
6. test_empty_input — empty batch → empty output
7. test_batch_boundary_32 — exact 32 inputs = 1 batch
8. test_batch_boundary_33 — 33 inputs = 2 batches (32+1)

All tests pass locally. Builds cleanly:

Updated INDEX.md:
- Added M2.x row to progress table (6/8 ✅, 2 ⬜)
- Updated total: 73 tasks, 48✅ + 2🟡 + 23⬜ (was 65 tasks)
- Updated gate count: 6/11 green (was 5/10)
- Test count: 247 passing, 2 ignored (was 239)

Blocks: M1.1 ✅ (already complete, unblocked)

tasks/M2.1-embeddings-client.md

@@ -4,7 +4,7 @@
 |---|---|
 | Phase | M2 — Projections |
 | Size | S — under 1 day |
-| Status | ⬜ Not started |
+| Status | ✅ Done |
 | Flags | — |
 | Spec | inlined below |
 | Blocks | M1.1 |