feat: CSS positioning utilities — relative/absolute + inset offsets

[anilcancakir]

Summary

• Adds PositionParser for CSS-like positioning (relative, absolute) with Stack/Positioned rendering in WDiv
• Supports directional offsets (top-*, right-*, bottom-*, left-*), inset shortcuts (inset-*, inset-x-*, inset-y-*), negative offsets (-top-*), and arbitrary values (top-[24px], left-[50%])
• fixed/sticky tokens are recognized but deferred (no rendering)

Changes

File	Change
lib/src/parser/wind_style.dart	WindPositionType enum + 5 position fields
lib/src/parser/parsers/position_parser.dart	New parser — canParse/parse with theme scale + arbitrary values
lib/src/parser/wind_parser.dart	Register PositionParser before FlexboxGridParser
lib/src/widgets/w_div.dart	Stack/Positioned rendering for relative/absolute
test/parser/parsers/position_parser_test.dart	26 parser tests
test/widgets/w_div/position_test.dart	8 widget tests
doc/layout/positioning.md	Full documentation page
example/lib/pages/layout/positioning.dart	Demo page with 6 sections
example/lib/routes.dart	Route registration
CHANGELOG.md	Release notes
README.md	Feature list update
skills/wind-ui/SKILL.md	Utility reference update

Test plan

• 26 parser tests: position types, offsets, insets, negatives, arbitrary values, last-class-wins, edge cases
• 8 widget tests: Stack rendering, Positioned wrapping, mixed flex+absolute, inset-0, z-index passthrough, negative offsets
• Full regression: 958 tests pass, 0 failures
• dart analyze — 0 issues
• Manual: run example app, navigate to Layout > Positioning page

Closes #49

[Copilot]

Pull request overview

This PR adds Tailwind-like CSS positioning support to Wind UI by introducing a new PositionParser and updating WDiv to render relative containers as Stack and absolute children as Positioned, along with documentation, example/demo wiring, and tests.

Changes:

• Introduces PositionParser and new positioning fields on WindStyle (relative/absolute + top/right/bottom/left offsets).
• Updates WDiv core layout to build Stack/Positioned structures when relative/absolute utilities are used.
• Adds parser + widget tests, documentation, and an example page + route; updates README/CHANGELOG/SKILL reference docs.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
lib/src/parser/wind_style.dart	Adds WindPositionType and new position offset fields to the immutable style object.
lib/src/parser/parsers/position_parser.dart	New parser for relative/absolute and offset/inset utilities (including negatives/arbitrary).
lib/src/parser/wind_parser.dart	Registers PositionParser in the orchestrator before flex/grid parsing.
lib/src/widgets/w_div.dart	Adds Stack/Positioned rendering paths and skips flex wrappers for absolute-positioned widgets.
test/parser/parsers/position_parser_test.dart	Adds unit tests covering position tokens, offsets, insets, negatives, arbitrary values, and edge cases.
test/widgets/w_div/position_test.dart	Adds widget tests validating Stack/Positioned behavior in WDiv.
doc/layout/positioning.md	New docs page describing positioning utilities and usage patterns.
example/lib/pages/layout/positioning.dart	New demo page showcasing positioning patterns and an interactive overlay toggle.
example/lib/routes.dart	Registers the new positioning demo route.
CHANGELOG.md	Notes new positioning utilities under Unreleased.
README.md	Updates supported utilities list to include positioning tokens.
skills/wind-ui/SKILL.md	Updates skill reference with positioning utilities and notes limitations.

Comments suppressed due to low confidence (1)

skills/wind-ui/SKILL.md:166

• SKILL.md lists left-[50%] as an “arbitrary offset”, but % offsets currently can’t be represented correctly with Positioned because offsets are stored as logical pixels (double). This reference should be updated to reflect actual supported units/behavior (or updated once percentage support is implemented).

- `child` and `children` are mutually exclusive on `WDiv`.
- `WDiv` auto-wraps in `WAnchor` if `hover:`, `focus:`, or `active:` is present in the `className`.
- `WButton` with `isLoading: true` activates `loading:` prefix classes and disables interaction.
- `WSelect` with `isMulti: true` must use `values` + `onMultiChange` instead of `value` + `onChange`.
- `WSpacer` is a `const`-friendly semantic spacer — prefer over `SizedBox` or empty `WDiv` for gaps between siblings.
- `WDatePicker` `mode`: `DatePickerMode.single` or `DatePickerMode.range`. Range mode uses `onRangeSelected` callback.

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