[guijacquemet]

Closes #97. Adds a public, pixel-free morphobricks_normalization.stain.check_options(preset, *, white=None, stain_vectors=None, polarizer=None). It runs every check to_scalar makes about these arguments, so consumers can refuse an unusable request before reading an image. This closes the one known limitation of fiberfeatures v0.6.0's RGB input (review J/K item R1).

What it checks, in to_scalar's order:

1. Unknown preset, refused with the known presets listed.
2. Presence rules: a missing needed option is refused, an unused option is refused (never ignored), and a polarizer must be in POLARIZERS.
3. white: three finite values in (0, 255]. A masked array (an all-false mask included) or a sequence holding np.ma.masked is refused before np.asarray.
4. Stain vectors (custom_od): names, three finite values each, norm ≥ 1e-6, masked values refused, collinear vectors (< 5°, sign-insensitive) refused, condition number > 50 refused. The default vectors go through the same checks.

One implementation:

• A private _checked_options() does the checks and returns the checked white and matrix. check_options wraps it, and to_scalar calls it directly to reuse the values, so each check runs once per call.
• _custom_od and _white_rescaled now receive checked values.
• Behaviour change: when both an option and the pixels are invalid, the option error is now reported first. Noted in the CHANGELOG.

Parity with main:

• 33 conversions (every preset, YXC/CYX, with and without white, default and user vectors, both polarizers, passthrough float and uint16) give identical images, exclusions and provenance (same SHA-256).
• For 40 refusal cases, the exception type and message text are identical.

Tests (tests/test_stain_check_options.py, NumPy only):

• 40 refusals called with no image, plus the same 40 through to_scalar, asserting identical messages;
• 17 accepted cases, at least one per preset;
• masked values refused before np.asarray;
• options refused before pixels;
• arguments not modified;
• check_options runs with scikit-image blocked.

Checks (tools/check.py, Python 3.12):

• With [dev,histology]: 2138 passed, 2 skipped (CSBDeep is installed only in its own CI job).
• Histology job files: 386 passed.
• Without the extra: 2077 passed, 63 skipped (scikit-image missing, as before).
• Not run locally: the CSBDeep parity job, the macOS and Windows matrix, Python 3.11 and 3.13, and the spec-pin job. CI covers them.

No version change. It ships in the next normalization release, with the coordinated pin move; fiberfeatures and MorphoFeature then call it at planning.

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