fix: matrix cache silently corrupts matrices when view/sensor/surface names collide

[silaskalmbach]

save_matrices keyed all per-entity matrices by bare entity name:
'{name}_sky_matrix' (TwoPhase, views AND sensors), '{name}_window_matrix'
(ThreePhase: views, sensors AND surfaces; FivePhase: views and sensors) and
'{name}_sun_direct_matrix' (FivePhase). When two groups share a name -- e.g.
a view and a sensor both named after the zone -- the later group overwrote
the earlier one in the .npz, and load_matrices restored the wrong array into
the survivor without any error. Downstream this manifests as degenerate
view matrices and noise-floor DGP values with no exception.

Namespace the keys per group (view_/sensor_/surface_) and stamp the file
with '_schema_version' = 2. Legacy (v1) files remain loadable when their
entity names do not collide; a colliding legacy file now raises ValueError
with a pointer to regenerate, instead of silently returning corrupt data.

Adds tests: roundtrips with colliding names for all three phase methods
(shapes and contents preserved), legacy-file compatibility, and the
legacy-collision error path.

[Copilot]

Pull request overview

Fixes silent matrix-cache corruption when view/sensor/surface entity names collide by namespacing cache keys per group and introducing a cache schema marker to distinguish legacy files.

Changes:

• Add schema v2 cache format with group-namespaced keys (view_, sensor_, surface_) and write _schema_version = 2 into newly saved caches.
• Update matrix-cache load paths to support both v2 caches and collision-free legacy (v1) caches; legacy caches with colliding names now raise ValueError instead of silently loading corrupt data.
• Add regression tests covering v2 roundtrips with colliding names across Two/Three/Five-phase methods plus legacy compatibility/error-path behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
frads/methods.py	Introduces schema v2 namespacing for matrix-cache keys, adds _schema_version, and updates load/save logic to avoid collisions and detect legacy caches.
tests/test_matrix_cache.py	Adds regression tests for collision-safe roundtrips and legacy cache handling.

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[taoning]

Thanks. I'd probably try to do something cleaner and bump the cache format that would require users to regenerate all the matrices. I will keep this open for now while working on a solution.