feat(mix_chart): add Mix-owned chart API

[leoafarias]

Related issue

No linked issue.

Description

Add mix_chart 0.0.1-beta.0: a Mix-owned, backend-neutral chart API for line/area, grouped/stacked/floating bar, and pie/donut charts. Consumers work with chart models, callbacks, Specs, generated Stylers, and contextual dot shorthand without importing or receiving fl_chart types.

fl_chart ^1.2.0 remains the private renderer. This keeps the mature layout, painting, and hit-testing engine while giving Mix control over the public design-system vocabulary, composition, interaction identity, semantics, and animation ownership.

Changes

• add immutable public data/configuration models, stable-ID hit results, axes, viewport controls, selection, cursors, and widget tooltips
• add generated Mix Specs/Stylers with factory, fluent instance, and contextual .axis(.label(...)) composition
• map line/area, grouped/stacked/floating bar, and pie/donut presentation through three private renderer adapters
• make animation ownership deterministic, snap incompatible topology changes, and honor reduced-motion settings
• harden dash validation, bar viewport/category handling, tooltip update lifecycle, and tap-outside dismissal
• provide root chart semantics, safe empty states, defensive validation, and tests for RTL, text scaling, interaction identity, controller ownership, and API boundaries
• rebuild the gallery, playground, dashboard, README, and pub.dev screenshot metadata around direct Flutter values and optional consumer-owned tokens
• add package-only mix_chart-v<version> release automation while leaving ordinary repository updates unpublished

Visual evidence

Analytics dashboard — dark

Chart galleries

Line gallery — light	Bar gallery — light

Interactive playground

Bar controls	Pie and donut controls

Validation

• melos run gen:build — passed
• melos run ci — passed across the repository
• melos run analyze — passed Dart analysis, DCM, and schema inventory
• dart format packages/mix_chart/lib packages/mix_chart/test packages/mix_chart/example/lib packages/mix_chart/example/test --set-exit-if-changed — passed
• flutter test in packages/mix_chart — all 50 tests passed
• clean hosted-dependency dart pub publish --dry-run — passed with 0 warnings
• exact committed-checkout dart pub publish --dry-run — passed with 0 warnings and only the expected monorepo override hints
• gallery routes rendered and screenshots were visually verified in a real browser at 1440 px widths

Review Checklist

• Testing: Unit, widget, interaction, semantics, adapter, API-boundary, publish-contract, and gallery smoke tests pass.
• Breaking Changes: None. The earlier branch-only adapter experiment was squashed out before review and had no published consumers.
• Documentation Updates: Package README, example documentation, gallery, release notes, and pub.dev screenshot metadata are updated.
• Website Updates: The standalone mix-docs repository is outside this PR; the package ships complete README and runnable gallery documentation.

Additional Information (optional)

mix_chart 0.0.1-beta.0 has been manually published to reserve the package name. Pub.dev GitHub Actions/OIDC is configured for conceptadev/mix, tag pattern mix_chart-v{{version}}, push events, and the required Production environment. Subsequent releases can use the isolated tag-triggered workflow added by this PR.

The package is still under legacy uploader ownership; transfer to the verified leoafarias.com publisher remains a separate irreversible admin action.

The 0.0.1-beta.0 scope intentionally excludes a native chart engine, renderer escape hatches, additional chart families, and per-mark spatial semantics. Those can be evaluated from concrete requirements without exposing the private backend now.