feat(package): sentinel, a spectral online anomaly spectrometer

Hierarchical online subspace anomaly detection for positionally
structured observation streams (e.g. IPv6 addresses), generic over
coordinate type, accumulator type, and domain bit-width.

The sentinel maintains per-cell low-rank linear subspace models and
scores each incoming batch via streaming thin SVD with exponential
forgetting. It produces raw statistical measurements — never opinions.

Core implementation:
- SpectralSentinel<C,V,N> engine with Sentinel128/Sentinel64 aliases
- G-V Graph integration for adaptive spatial partitioning
- Analysis selector (top-K competitive cells, Steiner-closed ancestry)
- SubspaceTracker per cell: Brand incremental SVD, EWMA baselines,
  CUSUM change detection, four scoring axes (novelty, displacement,
  surprise, coherence)
- Hierarchical coordination: SVD-based cross-cell scoring at internal
  g-tree nodes with per-member breakdowns
- Automatic depth-tiered noise injection for cold-start warm-up
- Spatial decay via configurable attenuation and depth selectivity
- Serde support behind a feature flag
- BatchReport/HealthReport/CellInspection public report types

21 ADRs documenting design decisions (ADR-S-001 through ADR-S-021).
Full algorithm specification and implementation status docs.

Three-level test suite:
- Unit tests inline and in src/tests/ (config, EWMA, CUSUM, tracker,
  convergence, observation, analysis set, variance, clipping)
- Crate tests in tests/ covering: API surface, ancestor chains,
  clip pressure, coordination, coverage matrix, decay, determinism,
  deferred warm-up, edge cases, graph routing, health, integration,
  invariants, noise injection, report structure, serde round-trips,
  spray resistance, suffix analysis, u64 path, and warm-up sequence
- Benchmarks (sentinel.rs)

Author: da2ce7

AGENTS.md

@@ -47,11 +47,12 @@ carries a **package qualifier** so the target document is never ambiguous.
 ADR references (`ADR-T-001`, `ADR-R-001`, …) are an exception — they
 use their own `ADR-<PREFIX>-<NNN>` form without the `§` prefix.
 
-| Prefix | Package              | Example document                 |
-| ------ | -------------------- | -------------------------------- |
-| `T-`   | Torrust (root crate) |                                  |
-| `M-`   | Mudlark              | `packages/mudlark/docs/idea.md`  |
-| `R-`   | render-text-as-image | `packages/render-text-as-image/` |
+| Prefix | Package              | Example document                      |
+| ------ | -------------------- | ------------------------------------- |
+| `T-`   | Torrust (root crate) |                                       |
+| `M-`   | Mudlark              | `packages/mudlark/docs/idea.md`       |
+| `R-`   | render-text-as-image | `packages/render-text-as-image/`      |
+| `S-`   | Sentinel             | `packages/sentinel/docs/algorithm.md` |
 
 ### General Rules