feat(cognitive-shader-driver): add Σ-codebook-index column to FingerprintColumns (B2)

[AdaWorldAPI]

Summary

Track-B PR 2. BindSpace SoA carries μ-fingerprints today (content /
cycle (Vsa16kF32) / topic / angle) but no Σ. Per Pillar-6 (PR #289)
and Σ-Codebook viability (#288, R²=0.9949 at k=256), every row should
also carry a Σ codebook index alongside its μ.

This PR adds the index column only — one byte per row indexing into a
256-entry Σ codebook.

What changes

• FingerprintColumns gains pub sigma: Box<[u8]>.
• FingerprintColumns::zeros(len) allocates vec![0u8; len].into_boxed_slice()
  for sigma alongside the four existing planes.
• New accessors:
  • pub fn sigma_at(&self, row: usize) -> u8
  • pub fn write_sigma(&mut self, row: usize, idx: u8)
• BindSpace::byte_footprint includes the new sigma byte per row.

What does not change

• The Σ codebook itself is not loaded here — that is a B3 concern.
  The codebook static + boot-load-from-disk lives in
  lance-graph-contract::sigma_propagation, contributed by B1.
  This PR only allocates the per-row index column so the SoA layout is
  ready when B3 wires the lookup in.
• No public API on BindSpace / BindSpaceBuilder changes outside the
  new struct field. Existing callers continue to work unmodified.

Cost

1 byte / row on the existing ~6.2 KB row footprint = 0.02 % growth.

Verified at len = 1:

component	bytes
content + topic + angle (3 × 2048)	6144
cycle (16 384 × f32)	65536
sigma (NEW, 1 × u8)	1
edge (u64)	8
qualia (18 × f32)	72
meta (u32)	4
temporal (u64)	8
expert (u16)	2
entity_type (u16)	2
total	71777 (was 71776)

The bindspace_footprint_adds_columns test asserts the new total
exactly so future drift trips a unit test.

Test plan

• cargo test --manifest-path crates/cognitive-shader-driver/Cargo.toml --lib bindspace:: — 12 / 12 pass on this branch
• sigma_column_zeros_initialised_to_index_zero — every row in a fresh FingerprintColumns::zeros(8) reads back 0
• sigma_column_round_trips_per_row — distinct indices (0, 17, 200, 255) round-trip through write_sigma / sigma_at; overwrite replaces (does not OR/accumulate)
• fingerprint_columns_sigma_len_matches_other_columns — implied row counts agree across all five planes for len ∈ {0, 1, 7, 64, 1000}
• bindspace_zeros_shapes — extended to assert bs.fingerprints.sigma.len() == 10
• bindspace_footprint_adds_columns — updated to 71777 (was 71776)

References

• PR jc: Σ-Codebook Viability Probe — empirically rules out CausalEdge64 8→16 byte expansion #288 — Σ-Codebook viability (R²=0.9949 at k=256)
• PR jc: Pillar 6 — EWA-Sandwich Σ-push-forward (cant-stop-thinking math foundation) #289 — Pillar-6: μ + Σ pairing on every BindSpace row
• B1 / B3 follow-ups: codebook static + boot-load in lance-graph-contract::sigma_propagation

https://claude.ai/code/session_01PjcbSTd9zdVPkta9qwkVKo