[Sad Catapella N3][Correctness] Direct Parameters/NormalizationParameters accept invalid offsets and scales #36

Baseline and classification

Normalization Batch 3 f784c95e3049483f87466150ffc5764375fa631d. Confirmed constructor validation gap by implementation tracing; no package tests executed. Both public parameter representations should agree on numeric invariants.

Evidence

• Parameters.__post_init__ lines 104–120 converts offsets/scales to float but checks only shape and channel indices. NaN, inf, zero and negative scales all pass.
• Parameters.apply lines 184–207 then divides using those values; to_columns lines 217–226 writes them without validation. In contrast from_columns lines 288–294 checks scale positivity and _number checks finiteness; consequently from_columns(to_columns(p)) == p is false for accepted direct objects.
• NormalizationParameters lines 33–40 likewise has no invariant validation; the normal fitting helper _parameters checks scale but not offset finiteness, so only callers using that helper receive even partial checks.
• Tests for Parameters cover channel count and order, not numeric validity.

Reproduction (source proof)

Parameters('z-score','image',None,True,(float('nan'),),(0.0,)) constructs successfully and .apply(np.array([2.])) returns nonfinite values rather than refusing the invalid normalization. p.to_columns() serializes nan and 0.0, but Parameters.from_columns(p.to_columns(), NormalizationSettings(method='zscore')) refuses these values.

Fix and acceptance

Validate offset finite and scale finite strictly positive upon construction for fluorescence; decide/document the RGB epsilon/shared-scale requirements. Apply the same invariant to NormalizationParameters. Preserve explicit nonfinite input pixels as nonfinite results; do not conflate them with invalid parameters. Reject invalid constructor arguments before application/serialization. Test NaN/±inf offset and scale, zero/negative scale, valid very small scale, fit-produced parameters and exact round trips. Keep messages naming channel and invalid value. No Numba use.