feat(remix): add segmented control component

[leoafarias]

Description

Adds RemixSegmentedControl<T extends Object>: a controlled, non-clearable single-select control with equal segment sizing, a persistent track, roving keyboard focus, RTL-aware navigation, and horizontal or vertical orientation.

This is stacked on Remix publish foundation #102.

Component API

• RemixSegmentedControl<T> accepts items, nullable selectedValue, non-null-emitting onChanged, enabled, orientation, loop, semanticLabel, excludeSemantics, style, and authoritative styleSpec.
• RemixSegmentedControlItem<T> accepts a stable non-null value, optional label/icon, required accessible name for icon-only items, per-item enabled/focus/autofocus state, and a per-item style.
• Activating the selected segment never clears it or emits another change.
• Horizontal keyboard order follows LTR/RTL; vertical controls use Up/Down. Home/End, Space/Enter, disabled-item skipping, wrapping/clamping, and one-tab-stop behavior are covered.
• Equal sizing works in bounded, unbounded, narrow, vertical, RTL, intrinsic-width, and 200% text-scale layouts.

Reuse and implementation

• Composes Naked's existing NakedToggleGroup and NakedToggleOption primitives for selection, focus, keyboard, pointer, and semantics behavior.
• Reuses Remix's RemixStyleSpecBuilder, StyleSpecBuilder, RemixBoxWithEffects, StyledText, and StyledIcon.
• It does not wrap RemixToggleGroup: segmented control has a persistent track, equal-segment geometry, and effects-aware item surfaces that are a different public anatomy.
• The equal-segment render object saturates finite arithmetic, rejects negative/NaN/infinite spacing, preserves positive child extents when gaps exceed available space, and keeps semantics/pointer bounds inside the track.
• Minimum and maximum main-axis intrinsics are computed separately from each child's corresponding intrinsic extent, so intrinsic-column parents can shrink and wrap labels without changing natural maximum sizing.

Mix/code-generation surface

• Both track and item anatomy use @MixableSpec.
• Track container and item container use generated forwarded BoxStyler APIs.
• mainAxisSize, track spacing, nested item, label, icon, and item spacing are generated fields.
• The only handwritten generated-surface override is SegmentedControlItemSpec.lerp, required so Remix box effects interpolate instead of snap-lerping an otherwise unrecognized custom spec type.
• No handwritten generated-style extension methods were added.
• Clean generation reproduced all 26 committed artifacts byte-for-byte.

No FortalSegmentedControl or fortalSegmentedControlStyle preset ships in v1. The docs now make explicit styling required, the icon-only example accepts a required SegmentedControlStyler, and the playground records why it hand-rolls its visual recipe. A Radix/Fortal preset remains a deliberate visual-design follow-up rather than an invented default in this correctness PR.

Visual Evidence

Captured and visually verified locally; the PNGs are ignored and are not part of the package diff:

• .context/screenshots/segmented-control-light.png — 388×344, SHA-256 b396d71c614c392e0930ff3a11d747fc47d77f97a3aab547b55178fb8407725b
• .context/screenshots/segmented-control-dark.png — 388×344, SHA-256 12f83a6f3aee012738506749c7b4b3ec30e680e23a167c01b59bae8db4c23aa9

Hosted GitHub attachment upload remains pending because the connector and CLI do not expose the issue-attachment endpoint.

Validation

• fvm flutter test packages/remix/test/components/segmented_control packages/remix/test/components/toggle_group packages/remix/test/public_api_test.dart packages/remix/test/public_api_compatibility_test.dart — 159/159 passed on hosted naked_ui 1.0.0-beta.9.
• The selected and unselected option contracts now assert isInMutuallyExclusiveGroup: true, matching Naked's segmented-control semantics.
• fvm dart run melos run generate:check — 26 generated artifacts reproduced byte-for-byte.
• fvm flutter analyze --fatal-infos — clean.
• Fortal parity verified the hosted naked_ui 1.0.0-beta.9 resolution while retaining the compatible ^1.0.0-beta.8 package constraint.
• GitHub test / Run tests for all packages on final head 3cd570db3 — passed.
• Formatting, conflict-marker scan, and git diff --check — clean.

naked_ui 1.0.0-beta.9 contains the mutually-exclusive ToggleOption semantics fix from conceptadev/naked_ui#84, and this branch's lockfile now resolves that hosted release.

Related Issues

• Resolved upstream by fix(naked-ui): correct selection semantics and press state naked_ui#84 and released in naked_ui 1.0.0-beta.9.

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Draft gates

• Public API, Naked/Remix reuse, Mix code-generation, and adversarial review complete.
• Equal-layout numeric and intrinsic hardening covered by regressions.
• Final focused and full repository CI pass on hosted naked_ui 1.0.0-beta.9.
• Light/dark screenshots captured and visually inspected.
• Missing Fortal preset documented as an explicit v1 visual-scope deferral.
• Naked UI refactor(remix): generate Fortal wrappers #84 is merged and released in a compatible prerelease.
• Mutually-exclusive semantics expectations are true and final CI passes on that dependency.
• Hosted screenshots are attached to this PR.
• Manual VoiceOver/TalkBack, web accessibility-tree, keyboard, RTL, narrow-width, and 200% text-scale checks are recorded.

Breaking Change

Does this PR require users of the package to manually update their code?

• Yes, this is a breaking change.
• No, this is not a breaking change.