feat(mix): add callable pressable builders across stylers

[leoafarias]

Related issue

No linked issue.

Description

Make compose → .pressable() → call the standard API for interactive Mix surfaces across Box, Text, Icon, Image, FlexBox, StackBox, WrapBox, and GridBox.

final button = BoxStyler()
    .padding(.all(16))
    .onPressed(.scale(0.96))
    .pressable();
button(onPress: save, child: const Text('Save'));

final heading = TextStyler().fontSize(18).pressable();
heading('Settings', onPress: openSettings);

Changes

• Add parameterless instance methods through typed private styler mixins and the existing generation hook, plus a direct method on GridBoxStyler. Replace this PR's earlier unreleased extension API.
• Return immutable callable builders that preserve unresolved styles and create a fresh Pressable with the appropriate styled child. Forward every Pressable option/default, preserve native content arguments, and keep keys on the outer widget. Builders expose no further styling or style override.
• Cover constructor parity, terminal type errors, generation, forwarding across all eight stylers, token/animation/state resolution, reuse, cancellation, feedback, keyboard and external checkbox/switch semantics.
• Document the convention in API guidance, examples and canonical skills. Include a source-dependent runnable example and a complete 31-call-site showcase migration audit.

Existing released Pressable, PressableBox, and ordinary styler APIs remain compatible. Layout-only stylers are unchanged. @MixWidget factories still return styles.

Review Checklist

• Testing: Validation details below.
• Breaking Changes: No changes to released constructor APIs; replaces only the unreleased extension in this PR.
• Documentation Updates: README, composition guidance, canonical skill references, local demo and migration audit updated.
• Website Updates: Separate btwld/mix-docs update is an explicit release follow-up.

Validation and release follow-ups

• melos run gen:build, melos run ci, and melos run format:check passed. Final CI includes 3,027 core Mix tests, 383 generator tests, and all dependent suites.
• Resolved analyzer signature/default parity, exact terminal compile errors, and generation-hook tests passed. Focused interaction coverage includes context-dependent tokens, animation, keyboard, feedback, cancellation and semantics.
• Published-dependency showcase: analysis clean, all 129 tests passed, web build passed. The new local source-dependent example also builds for web.
• melos run analyze: schema inventory and Dart analysis passed (existing informational notices remain). DCM still reports exactly four pre-existing findings in unchanged files: member ordering and sealed-switch wildcard in style_animation_builder.dart:81,90; argument ordering in decoration_style_mixin.dart:273,359. No new DCM findings remain.
• Canonical and installed Mix/microinteraction skills validated; code and the complete examples/docs/skills audit independently reviewed.

The hosted showcase deliberately uses published Mix in CI. Keep its current constructors, including standalone DartPad Snacks, until a release includes callable builders. The new source-only example demonstrates the API now; guides/pressable-builders-release-audit.md maps all 22 PressableBox and 9 Pressable call sites and records hit-area, nested-control, keyboard and semantics constraints for migration.

Plan and review records

• Implementation plan
• Review and validation record

Both records are committed with this PR. Repository instructions now require plans and relevant review/audit records to be committed and pushed with future PRs.