hdr: run Cascade on ndarray's rolling floor and σ-lattice

[AdaWorldAPI]

What changes

graph/blasgraph/hdr.rs now consumes ndarray::hpc::rolling_floor (AdaWorldAPI/ndarray#328):

• Moments and drift: exact MomentsU32 moments, the reservoir and drift detection come from ndarray. ShiftAlert and ReservoirSample are re-exports.
• Threshold derivation: band cuts (3σ / 2σ / 1σ / μ) and cascade cuts (1σ to 3σ in quarter-σ steps) are SigmaLevel points. The floor's current shape and running coordinates answer them per call, and nothing is cached.
• Unchanged API: the public API stays the same: band, expose, thresholds, cascade_thresholds, cascade_at, query, observe, observe_batch and recalibrate.

Semantic deltas

• Running coordinates: thresholds now follow the running (μ, σ). Before, they were frozen at the last recalibration.
• Empirical band ranks: these now use the Gaussian tail of each σ level (1587, 228 and 13 per 10000) instead of the coarse 0.159, 0.023 and 0.001. Across reservoir lengths 1..=1000, the chosen sample index changes at:
  • 150 lengths for 1σ (159→158 at len 1000);
  • 98 lengths for 2σ (23→22);
  • 230 lengths for 3σ (1→1 at len 1000).
• Empirical cascade ranks: unchanged at every length.
• cascade_at(k): now uses the same shape-aware lookup. Before, it applied the anchor's σ arithmetic even when the empirical shape was active.

Tests

All 191 blasgraph tests pass with no re-pins. Clippy -D warnings and fmt are clean.

CI stays red until ndarray#328 merges, because CI builds against ndarray master.

Scope fence

The diff contains 0 matches for ZGamma, zspace, Fisher, hyperbolic, Palette256, FisherZTable, bgz or Mexican.

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• crates/lance-graph/src/graph/blasgraph/hdr.rs

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[AdaWorldAPI]

member-tests fails with unresolved import ndarray::hpc::rolling_floor (4 errors, all in hdr.rs).

CI builds against ndarray master, and hpc::rolling_floor does not exist there until AdaWorldAPI/ndarray#328 merges. This is the ordering dependency stated in the description, not a defect in this diff. Built against the #328 branch, cargo test -p lance-graph --lib -- graph::blasgraph passes 191/191 locally, and clippy -D warnings is clean.

No fix is needed on this branch. Once #328 is on ndarray master, a CI re-run here should pass.

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