runtime: let signal enter as geometry, not as prose about signal

No ingest path could carry a vector. engram_node/_full/_layered take text
only, and a node acquired an embedding solely via engram_embed_backfill
DERIVING one from n->content. That made text the mandatory entry medium:
any non-text modality had to be described in prose first, so the geometry
we then reasoned over was the geometry OF THE DESCRIPTION, not of the
signal. Measured: POST /api/nodes accepted an "emb" field, returned 200
with a fresh id, and stored nothing — emb_dim=None, embedded=false.

engram_node_set_emb attaches a vector to an existing node. Off-dimension
vectors are stored but not indexed (the HNSW build loop already filters on
emb_dim), so modality geometry is durable and addressable without
perturbing the canonical index. Setting emb also makes the node ineligible
for embed_backfill, so a realizer's vector is never overwritten by a
text-derived one.

Two reporting fixes ride along, because both are how the drop stayed
invisible: the create response now reports emb_set instead of being
success-shaped regardless, and the node document now always emits emb_dim
and embedded — without which a genuine ingest drop and a mere reporting
gap are indistinguishable.

Verified live: voice node emb_dim=64 embedded=true; text control emb_dim=0
embedded=false; malformed hex, length mismatch and dim<=0 all reject.

KNOWN PLACEMENT DEFECT: this is at the consumer. Ingest is a language
concern, not an engram feature — every el program touching any modality
needs it. The vector also marshals as a hex STRING because el has no
first-class geometry value, which reintroduces text as the transport
medium one layer below the problem being fixed. The durable shape is
geometry as an el value plus declarable realizers, after which the engram
stops having an ingest concept at all. Landing this as the verified probe
that proves the path.

lang/runtime/el_runtime.h

@@ -613,6 +613,11 @@ void      engram_strengthen(el_val_t node_id);
 void      engram_forget(el_val_t node_id);
 el_val_t  engram_prune_telemetry(el_val_t older_than_ms);
 el_val_t  engram_node_count(void);
+/* Attach geometry to an existing node. `hex` is little-endian float32,
+ * exactly dim*8 hex chars — the encoding realizers already emit. Lets a
+ * non-text modality enter as geometry instead of being described in prose
+ * and embedded as its description. Returns 1 on success, 0 otherwise. */
+el_val_t  engram_node_set_emb(el_val_t id, el_val_t hex, el_val_t dim);
 el_val_t  engram_search(el_val_t query, el_val_t limit);
 el_val_t  engram_scan_nodes(el_val_t limit, el_val_t offset);
 void      engram_connect(el_val_t from_id, el_val_t to_id, el_val_t weight, el_val_t relation);