Establishes the kb-embed trait crate so concrete embedding adapters
(p3-2 fastembed, future ollama-embed/candle) target a stable surface.
Pure re-export of kb_core::{Embedder, EmbeddingInput, EmbeddingKind,
EmbeddingModelId, EmbeddingVersion} plus a feature-gated deterministic
mock for downstream tests.

MockEmbedder (cfg(feature = "mock"), default OFF):
- Per-component hash recipe: blake3(seed_le8 || kind_byte ||
  text_len_le8 || text || i_le8). Length-prefixed text avoids the
  domain-separation ambiguity where two (text, i) pairs could shift
  bytes between text tail and the i field.
- Document = 0u8, Query = 1u8 — same text different kind yields
  different vectors (mirrors e5 prefix behaviour).
- Per component: blake3 first 8 bytes → u64 → reinterpret as i64 →
  f64/i64::MAX → f32. i64::MIN gives -1.0000000000000002 which f32
  rounds to -1.0; range [-1, 1] holds.
- L2 unit-normalised. Norm sums in f64 (avoid catastrophic precision
  loss) before f32 cast. Zero-norm guard skips the divide.
- with_seed(...) constructor lets two embedders share identity but
  produce different vectors — useful for downstream parametric tests.

Helpers:
- assert_vector_shape(vecs, dims) — len + finite check.
- assert_unit_norm(vecs, tolerance) — caller-supplied tolerance;
  5e-4 documented as safe for dims=384 under f32 epsilon × √dims.

Tests:
- cargo test -p kb-embed (no features): 2 reexport/dyn-dispatch tests.
- cargo test -p kb-embed --features mock: 7 tests including 100-case
  proptest asserting len == dims, all finite, ‖v‖ ≈ 1.0 within
  tolerance, Doc(text) byte-equal Doc(text), Doc(text) ≠ Query(text),
  Doc(text1) ≠ Doc(text2).
- All 220 workspace tests pass; clippy clean for both default and
  mock-on feature configurations.

Symbol gating: nm on the release rlib confirms zero MockEmbedder
symbols under default features; three trait impl symbols under
--features mock. Spec invariant "release builds MUST NOT include
MockEmbedder" verified at the symbol level.

Allowed deps respected: kb-core, kb-config, serde, thiserror, tracing,
plus anyhow (forced by trait return type) and blake3 (justified by
the determinism contract; already in workspace lockfile via kb-core).
No fastembed/ort/tokenizers anywhere.

Out of scope: real adapter (p3-2), reranker traits (P+).

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

@@ -0,0 +19,4 @@
+# workspace lockfile (transitively via kb-core); pulling it in here adds zero
+# build cost and keeps Cargo.toml simple.
+blake3     = { workspace = true }
+

@@ -0,0 +67,4 @@
+            (norm - 1.0).abs(),
+        );
+    }
+}

@@ -0,0 +92,4 @@
+        hasher.update(&self.seed.to_le_bytes());
+        hasher.update(&[Self::kind_byte(kind)]);
+        // Length-prefix `text` (LE u64) so the boundary between `text` and the
+        // trailing `i` field is unambiguous — without this, `("ABCDEFGH", 0)`

@@ -0,0 +106,4 @@
+        // Map to [-1.0, 1.0]. `i64::MAX` is finite in f64 so the ratio is
+        // always finite. Casting back to f32 cannot produce a NaN/Inf for
+        // values in this range.
+        // Note: i64::MIN/i64::MAX gives -1.0000000000000002 → f32 cast rounds to -1.0; range [-1, 1] holds in f32 even with this asymmetry.

@@ -0,0 +122,4 @@
+
+    fn dimensions(&self) -> usize {
+        self.dimensions
+    }

@@ -0,0 +167,4 @@
+            .unwrap();
+        prop_assert_ne!(&doc_a, &q, "Doc(text) must differ from Query(text)");
+
+        // Text differential: Doc(text) != Doc(text2) when text != text2.

@@ -0,0 +29,4 @@
+    fn embed(&self, inputs: &[EmbeddingInput<'_>]) -> anyhow::Result<Vec<Vec<f32>>> {
+        Ok(inputs.iter().map(|_| vec![0.0; self.dims]).collect())
+    }
+}