feat(mix_generator): derive nested styler types for StyleSpec fields by convention

[leoafarias]

Problem

A @MixableSpec field typed StyleSpec<XSpec> only gets a styler-typed API (XStyler constructor param, setter, field factory, Prop.maybeMix merge semantics) when annotated @MixableField(setterType: XStyler). That override cannot work for same-package generated stylers: annotation type arguments are resolved by the analyzer while the generator runs, and build phases hide later-phase outputs from the resolver — so the type never resolves, setterType silently reads as unset, and the field degrades to a raw StyleSpec param with Prop.maybe value semantics (replacement instead of Mix merge accumulation). No error, wrong merge behavior.

This is the standard downstream-aggregator shape: any package composing its own specs (e.g. SuperDeck's SlideSpec nesting MarkdownAlertSpec, whose stylers this same generator emits) hits it. Reproduced end-to-end in a scratch package before writing this — first build and every rebuild silently degrade.

Change

Derive the nested styler type by the same XSpec → XStyler naming convention that already names generated styler classes (deriveStylerName). Resolution order in SpecStylerClassBuilder:

setterType override → curated list mix → curated mixTypeFor → StyleSpec<XSpec> convention (new) → raw fallback

The derivation is pure string manipulation by design: it needs no analyzer resolution, so it works for same-package generated stylers. It cannot be validated at generation time (that's the point), so a non-conventional styler name surfaces as an analyzer error in the generated output — loud, at compile time — rather than a silent semantic change. setterType still overrides the convention for resolvable types.

Breaking

Specs with bare StyleSpec<XSpec> fields previously generated raw StyleSpec constructor params; those now take XStyler. Call sites passing resolved StyleSpec values must pass the styler instead. Flagged in the changelog under the unreleased 2.1.2.

Verification

• All 292 mix_generator tests pass; two new smoke tests cover the convention, including the same-package case where the nested styler is emitted by this same builder run and can never resolve.
• Existing setterType tests unchanged and passing (override path intact).
• Regenerating packages/mix (78 outputs) produces zero diff — FlexBoxStyler/StackBoxStyler keep their curated compound surfaces (partial-match test updated: nested field now gets its derived styler type, compound flattening stays inactive).
• dart analyze + dart format clean on mix_generator and mix_annotations.

Docs

MixableField.setterType docstring now explains the convention and when the override is still needed.

Downstream

With this in a 2.1.2 release, conceptadev/superdeck#98 can drop its legacy @MixableStyler hand-written aggregator stylers and every @MixableField(setterType:) annotation, going fully spec-driven.