Type filter predicates and colorConfig to eliminate any in filter-config (toward #133)

[Epi-1120]

Purpose

Incremental step toward #133 — eliminate implicit and explicit any from src/. This PR narrows the scope to src/filter-config.ts and its unit test only.

Under tsc --noImplicitAny --strictNullChecks this file produces 9 implicit-any errors (on the filterPredicates callbacks and the two inner association callbacks). It also carries the single explicit any on the exported colorConfig. This PR eliminates all 10.

Approach

• The filterPredicates callbacks receive, at runtime, the transformed variants produced by ./adapters/variation-adapter (TransformedVariant = VariationDatum & Variant) — e.g. they read association, clinicalSignificances, sourceType, which are on Variant but not on VariationDatum. The any was previously masking this mismatch.
• Introduce a local FilterPredicate alias and tighten filterPredicates, VariantsForFilter, getFilteredVariants's callback, and countVariantsForFilter's variant parameter to TransformedVariant.
• Keep the exported colorConfig signature as (variant: VariationDatum) => string so it remains assignable to NightingaleVariation.colorConfig (used at src/protvista-uniprot.ts:327). The narrowing TransformedVariant cast is localised to colorConfig's body and documented inline.
• The existing unit test already uses transformedVariantPositions as VariantsForFilter. With VariantsForFilter now demanding the richer shape, that single-step cast no longer structurally overlaps (hand-built mocks are intentionally minimal, e.g. begin: number instead of string). Upgrade it to as unknown as VariantsForFilter — the TypeScript-recommended form — which preserves the test's intent without changing test data or runtime behaviour.

Per the issue's guidance to land this incrementally, noImplicitAny/strictNullChecks and the @typescript-eslint/no-explicit-any rule are not flipped on yet — that belongs in the closing PR after every directory has been cleaned.

Testing

• npx eslint src --ext .ts — clean.
• npx tsc --noEmit --noImplicitAny --strictNullChecks — all 9 previously reported errors from src/filter-config.ts are gone; no new errors surfaced anywhere else (total project errors: 98 → 89).
• npx vitest run src/__spec__/filter-config.spec.ts — 5/5 tests pass (all four colorConfig colour-per-variant assertions plus the getFilteredVariants callback test).
• npx vite build — successful; declaration emission and bundle unchanged.

Visual changes

None — type-system-only change.

Checklist

• My PR is scoped properly, and "does one thing only"
• I have reviewed my own code
• I have checked that linting checks pass and type safety is respected
• I have checked that tests pass and coverage has at least improved, and if not explained the reasons why
• If needed, the changes have been previewed by all interested parties.