feat: add custom themes and switch to oklch

.changeset/base-styles.md

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+---
+'@fuzdev/fuz_css': minor
+---
+
+feat: rework base styles - interaction states, micro-surface variables, body font, button border style, section rhythm
+
+Breaking:
+
+- `body` reads `--font_family` (default `var(--font_family_sans)`) instead
+  of `--font_family_sans`; set `--font_family` for a serif body.
+- Buttons read `--button_border_style` (default `var(--border_style)`) and
+  `--button_border_style_active` while pressed. A contextual
+  `--border_style` on an ancestor no longer reaches buttons - set the
+  button knobs there too.
+- Selected buttons use `--text_00` for inverse text (was
+  `--text_05`/`--text_10`); `.palette_X` buttons fill with `palette_X_50`
+  (was stop 40), the selected border matching the fill.
+- Hovering an input, textarea, or select colors the border with
+  `--outline_color` instead of `--border_color_20`; disabled inputs no
+  longer react to hover.
+- `section` bottom margin is a `--flow_margin` multiple (same default),
+  scaled by size composites; `.unstyled` opts out.
+
+New:
+
+- Focus outlines use `outline-offset: var(--outline_offset)` (default
+  `1px`).
+- Themable micro-surfaces: `--scrollbar_thumb_color` (`var(--shade_40)`),
+  `--scrollbar_track_color` (transparent), `--caret_color`
+  (`var(--accent_50)`), `--backdrop_color` (`var(--darken_60)`).
+- `@media (prefers-contrast: more)` maps onto the curve knobs in the
+  `fuz.preferences` layer; theme overrides beat it.
+- `check_theme` gains `GATE_SELECTED_TEXT`; the knob catalog's border
+  styles gain `inset`/`outset`.

.changeset/generators.md

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+---
+'@fuzdev/fuz_css': minor
+---
+
+feat: dev-server prescan, and `base_css` without `variables` is an error
+
+- The Vite plugin pre-scans sources at dev-server startup so the first page
+  load has complete utility CSS. New `prescan` option: `true` (default,
+  `src` under the Vite root), `false`, or an array of directories.
+- `base_css` enabled with `variables: null` is now the error diagnostic
+  `theme_variables_disabled`. Set `base_css: null` too for utility-only
+  mode, or keep `variables` and set `additional_variables: 'all'` to bundle
+  the full theme.
+- `CssResolutionResult` loses `referenced_variables`.

.changeset/oklch-color-system.md

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+---
+'@fuzdev/fuz_css': minor
+---
+
+feat: derived OKLCH color system with semantic intents and cascade layers
+
+Breaking:
+
+- `--color_X_NN` → `--palette_X_NN` (10 letters × 13 stops);
+  `ColorVariant`/`color_variants` → `PaletteVariant`/`palette_variants`.
+- Class renames: `border_color_X_NN` → `border_X_NN`,
+  `outline_color_X_NN` → `outline_X_NN`, `shadow_color_X_NN` →
+  `shadow_X_NN`, `.color_a`-`.color_j` → `.palette_a`-`.palette_j`.
+  `.color_X_NN`, `bg_X_NN`, `border_color_NN`, and `shadow_color_*` keep
+  their names.
+- Classes removed: `.fg_NN`/`.bg_NN` (use
+  `background-color:var(--fg_10)`; `bg_` is now the opaque prefix),
+  `.hue_a`-`.hue_j` and `--hue`, and every `_light`/`_dark` variable and
+  class.
+- `--hue_a`…`--hue_j` are OKLCH angles (blue `250`, was `210`); replace
+  `hsl(var(--hue_x) …)` with `oklch(<l> <c> var(--hue_x))` or a stop.
+- `--tint_hue`/`--tint_saturation` → `--hue_neutral` + `--neutral_chroma`.
+- `color-mix()` interpolates `in oklab` (was `in hsl`).
+- Shipped CSS is layered `fuz.base` < `fuz.preferences` < `fuz.theme` <
+  `fuz.utilities`; unlayered consumer styles beat all of it. Custom
+  `base_css` is re-layered into `fuz.base`.
+- `variables.ts` exports only `default_variables`; read a variable with
+  `default_variables.find((v) => v.name === 'space_md')`. `icon_sizes` →
+  `ICON_SIZES` (`ICON_SIZES.xs === 18`, was `'18px'`); `Z_INDEX_MAX`
+  removed (inline `2147483647`).
+
+New:
+
+- Curve knobs: `--chroma_scale`, `--palette_lightness_00/_100/_curve` (and
+  `shade_`/`text_`), `--palette_chroma_min/_max`, `--chroma_curve`, with
+  pinnable derived stops `--palette_lightness_NN`, `--palette_chroma_NN`,
+  `--chroma_shape_NN`.
+- Intent knobs `--hue_accent`/`_positive`/`_negative`/`_caution`/`_info`,
+  each with a 13-stop scale (`--accent_00`…`--accent_100`), token classes
+  (`.positive_50`, `.bg_caution_10`), `--selection_color`, and
+  `intent_variants`/`IntentVariant`. Links, focus, selection,
+  `accent-color`, and disabled-active feedback use them; focus follows
+  `--outline_color` with the accent as fallback.
+- Per-slot chroma multipliers `--palette_X_chroma_scale` and
+  `--<intent>_chroma_scale` (default `1`; brown `f` ships at `0.55`).
+- `--border_color_lightness`/`--border_color_chroma` derive the
+  `border_color_*` alpha ramp through the neutral intent.
+- Value tables in `variable_data.ts`: `FONT_SIZES`, `SPACE_SIZES`,
+  `BORDER_RADII`, `DISTANCES`, `LINE_HEIGHTS`, `DURATIONS`
+  (+ `duration_variants`), `SHADOW_GEOMETRY`, `SHADOW_ALPHAS`,
+  `OVERLAY_ALPHAS`.
+- Design-time modules `ramps.ts`, `oklch.ts`, `wcag.ts`.

.changeset/theme-authoring.md

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+---
+'@fuzdev/fuz_css': minor
+---
+
+feat: themes as knob-sets - schema, registry and modifiers, scheme stance, knob catalog, checks, pure renderer, build-time `theme` option
+
+Breaking:
+
+- `Theme` moves to `variable.ts` as a strict zod schema:
+  `import type {Theme} from '@fuzdev/fuz_css/variable.ts'` (was
+  `theme.ts`). Unknown properties are errors; `parse_theme(value)` returns
+  a theme-or-`null`.
+- `RenderThemeStyleOptions.empty_default_theme` removed - pass the defaults:
+  `render_theme_style({name: 'base', variables: default_variables})`.
+- `render_theme_style` loses `specificity` and gains
+  `layer?: string | null` (default `'fuz.theme'`); `generate_theme_css`
+  loses its specificity parameter; the `theme_specificity` generator option
+  is removed.
+- `default_themes` is just base. Low/high contrast are `contrast_modifiers`,
+  composed over a theme with `compose_themes(base, ...overlays)`.
+
+New:
+
+- `Theme.scheme?: 'dual' | 'light' | 'dark'` (`ThemeScheme`). Author a
+  single-scheme theme single-slot and pass it through
+  `resolve_theme_stance` (`theme_stance.ts`), which fills `scheme_mirror`;
+  the renderer pins `color-scheme`.
+- Exemplar themes under `themes/`: smolder, parchment, concrete, nineties,
+  phosphor (dark-only), neon (dark-only).
+- Scale knobs `--shadow_alpha_scale`, `--radius_scale`, `--scale_factor`,
+  `--font_weight`, `--heading_font_weight` (hook; setting it flattens the
+  ladder), `--heading_font_family`, `--background_image`.
+- `knobs.ts`: the typed knob catalog (`theme_knobs`, `theme_knob_by_name`,
+  `theme_knob_axes`); `palette_glosses` in `variable_data.ts`.
+- `theme_check.ts`: `validate_theme(unknown)`, `check_theme` (gamut,
+  monotonicity, contrast gates; `GATE_*` thresholds), `compile_theme`
+  (per-theme chroma caps), `create_theme_resolver`,
+  `known_theme_variable_names`.
+- Generators take `theme`, baked into the output and tree-shaken:
+  `vite_plugin_fuz_css({theme: phosphor_theme})`. Composes with fuz_ui's
+  `ThemeRoot`, the runtime theme winning. `apply_theme_variables` is
+  exported from `variable_graph.ts`.
+- `theme.ts` no longer imports `variables.ts` (~1.3KB minified, was ~38KB).