feat(createModel): add selection state layer between registry and selection

[johnleider]

Summary

• Introduces createModel as the shared selection state foundation between createRegistry and createSelection/createSlider
• Refactors createSelection to extend createModel instead of reimplementing selection logic (~90 lines removed)
• Generalizes useProxyModel with a structural ProxyModelTarget interface, removing hard dependency on SelectionContext
• Adds full documentation page with architecture diagram, reactivity table, and live examples

Architecture

createRegistry → createModel → createSelection → createSingle/createGroup/createStep
                             → createSlider (on feat/slider-component)

createModel provides: selectedIds, select/unselect/toggle, mandatory enforcement, disabled guards, apply bridge for v-model sync, and reset.

Files Changed

File	Change
composables/createModel/index.ts	New composable
composables/createSelection/index.ts	Refactored to extend createModel
composables/useProxyModel/index.ts	Generalized with ProxyModelTarget
composables/index.ts	Added barrel export
docs/pages/composables/selection/create-model.md	New docs page
docs/examples/composables/create-model/	4 example files
docs/pages/composables/index.md	Added to Selection table
docs/pages/composables/selection/create-selection.md	Updated architecture diagram

Test plan

• All 3005 existing tests pass (no regressions)
• Typecheck clean
• Lint clean
• Verify docs page renders correctly in dev server
• Verify live examples work (color swatch selection)

[pkg-pr-new]

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commit: b1d34c3