kernel(validate): synthetic negative-path fixture; drop vacuous slug check

Task 14 review, findings 1 and 2.

Finding 1: the only tests against Validate exercised the clean path against
real EF data, so the evidence that each check can actually fire lived in a
scratch mutation probe that was never committed. A future edit that quietly
weakened a check would leave the suite green, since a weaker check only
makes more inputs pass. Added a small synthetic two-season, two-rank rite
fixture in test_validate.ml -- not EF -- letting each test violate exactly
one invariant directly: a temporal that raises (coverage), a season that
recurs (seasons), a week that decreases mid-run (week), a weekday that
disagrees with Date.weekday (weekday), a rank absent from the declared vocab
(rank), and a colour outside Colour.all (colour, via a same-representation
Obj.magic value, safe here because the check compares by structural equality
rather than pattern match). A clean-baseline test confirms the fixture itself
reports zero failures before any mutation is applied.

Each new test was verified non-vacuous by temporarily weakening its
corresponding check in validate.ml, confirming the matching test fails, then
reverting -- the same trap one level up, checked explicitly rather than
assumed.

Finding 2: removed the slug well-formedness check. Slug.t is a private string
validated on every construction path, and to_string is the identity, so
round-tripping an existing Slug.t can never fail -- the check was structurally
incapable of firing. Folded the explanation into the comment block that
already covers why slug uniqueness isn't checked, since it's the same kind of
fact: a property the type system delivers, not one Validate needs to assert.
The colour check stays: unlike slug, Colour.all is a hand-maintained list
that can drift from the type, so it is only practically (not structurally)
tautological, the same class as the rank closure check.

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