refactor: generate Mix styling layer with mix_generator

[leoafarias]

Intent

Replace ~1,450 lines of hand-written Mix Spec/Styler boilerplate (copyWith, lerp, resolve, merge, props, debugFillProperties) with mix_generator output — the same codegen Mix 2.x uses for its own specs. Net −1,160 hand-written lines in the styling layer; what remains by hand is only what carries meaning (field declarations, constructor defaults, SlideSpec.of, toStyle()).

Architecture

Two tiers, split for a reason proven empirically before committing:

• Leaf specs (alert type, blockquote, checkbox, codeblock, list, table): fully spec-driven — @MixableSpec generates the entire *Styler class, including a fluent API these classes never had (per-field setters, field factories, onHovered/onFocused variant helpers).
• Aggregators (slide.dart, markdown_alert.dart): generated spec mixin + hand-written styler (fields + two constructors) on the legacy @MixableStyler mixin. Forced: nested custom specs need @MixableField(setterType: MarkdownAlertStyler), but annotation type arguments cannot reference same-package generated classes — build phases hide them from the resolver and the generator silently degrades to value semantics (replacement instead of Mix merge accumulation). Hand-written constructors can reference generated classes freely, so the hybrid keeps correct Prop.maybeMix semantics. Rationale recorded in build.yaml and both file headers.

Breaking changes

• Canonical styler names are now *Styler (SlideStyler, MarkdownAlertStyler, ...) per the Mix idiom; old *Style names remain as typedef aliases.
• TextStyle-typed styler params are now TextStyleMix (wrap existing values with TextStyleMix.value(...)); these fields gain Mix field-wise merge semantics through the style cascade.

Also in this PR

• Drop vestigial custom_lint dev dep (no plugin configured anywhere; it pinned analyzer <9 and hard-blocked mix_generator).
• Changelog the MarkdownText* deprecation that shipped unchangelogged in fix: apply SlideSpec styles to inline Markdown #97.
• Root CHANGELOG.md was a drifted copy of the package changelog; it now points at per-package changelogs.

Impacted packages

superdeck (styling, deck, markdown, tests), demo, playground, plugins/pdf (rename only), docs.

Verification

• melos run analyze (dart analyze + DCM, fatal warnings) green across all packages
• flutter test: superdeck 654/654, pdf plugin 27/27 — goldens included
• .g.dart output regenerates byte-identically from a clean build cache in a single pass, with zero degraded setters (grep-verified Prop.maybeMix on all nested spec fields)

Follow-up

mix_generator could derive nested StyleSpec<XSpec> setter types by the same XSpec → XStyler naming convention it already uses for class names — pure string derivation, no analyzer resolution — which would let the two aggregators go fully spec-driven and delete every setterType: annotation. Upstream PR against btwld/mix incoming.

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[leoafarias]

Upstream generator change is up: btwld/mix#961 — once it ships in mix_generator 2.1.2, the two aggregator stylers here (SlideStyler, MarkdownAlertStyler) can drop the legacy @MixableStyler path and every @MixableField(setterType:) annotation, going fully spec-driven.