feat(style): reader style overrides via a styleOverrides viewer prop

[dqnykamp]

Add reader (end-user) style overrides: a styleOverrides prop on <DoenetViewer>/<DoenetEditor> that lets a host application remap what each style number looks like on behalf of a reader — e.g. colors a color-blind reader can better distinguish. This realizes the accessibility goal behind style numbers: the reader's remapping wins, every object of that style shifts together, and the author's distinctions survive.

Stacked on #1523 (#1522 is merged) — merge #1523 first; this PR's diff includes its commits until then (the branch is rebased onto the current style-palette-component head).

Format

<DoenetViewer
    doenetML={source}
    styleOverrides={{
        styles: {
            1: { lineColor: "#0072b2", markerColor: "#0072b2" },
            2: { lineColor: "#d55e00", lineWidth: 6 },
        },
    }}
/>

ReaderStyleOverrides (exported from @doenet/utils, re-exported by @doenet/doenetml and @doenet/doenetml-iframe) is plain JSON: every style-definition key is overridable except the *Word descriptors. A reader can also select one of the built-in palettes wholesale via a palette field (case-insensitive name, validated against the registry — resolveReaderPaletteName is exported): the reader's palette replaces authored <stylePalette>/<styleDefinition> styling everywhere, style numbers beyond its size cycle onto it (taking precedence over the authored palette's cycle size — essential when a reader picks the four-style grayscale palette), and styles overrides apply on top. Unregistered names are silently ignored.

Palette discovery for hosts. So a host can render a palette picker with swatches, getStylePalettes() / getStylePalette(name) return each palette's name, description, and per-style resolved definitions (colors for both themes, line/marker settings, and the curated color words for accessible swatch labels) as plain JSON. Exported from @doenet/doenetml; the standalone bundle also installs window.getDoenetStylePalettes / window.getDoenetStylePalette for CDN pages. @doenet/doenetml-iframe adds an onStylePalettes callback to <DoenetViewer>/<DoenetEditor> that reports the palettes of the bundle actually booted inside the iframe — the wrapper and bundle ship separately, so the pinned version is the only correct source — feature-detecting the bundle global and reporting null for bundles too old to provide it.

Behavior

• Overrides apply per-key as the final layer of every section's merged style definitions — after authored <styleDefinition>s/<stylePalette>s and after the contrast diagnostics, so reader values win over everything authored, never emit author-facing diagnostics, and cannot mask diagnostics about authored values (they are also stored positionless as a second guard).
• *Word keys are rejected and always re-derived, keeping core-computed style descriptions ("thick red line") truthful for reader-chosen colors; a missing dark-mode color is derived from the reader's light color with the same accessibility-aware derivation authored styles get.
• Plumbing mirrors the theme/darkMode precedent: initial value rides in the generateJavascriptDast args; live prop changes push a setStyleOverrides action into a UI-only, never-saved readerStyleOverrides state variable on the document (mirrored into sections). The iframe wrapper forwards the prop through its existing serializable-props path with no changes.
• The editor's rendered preview applies the overrides; the code pane and context help keep showing authored truth.
• Authored per-component non-color attributes (e.g. <point markerSize="8">) still win over reader overrides in v1; colors — the accessibility-critical channel — are fully reader-controlled since no per-component color attributes exist.

Tests

• Worker (readerStyleOverrides.test.ts): init-arg overrides beating document- and section-level <styleDefinition>s and palettes, hex → color-word/dark-mode derivation, sanitization of word/unknown keys, live setStyleOverrides update and clear, no accessibility diagnostics from deliberately low-contrast reader values, and reader palette selection (replaces authored styling, cycles style numbers onto the reader palette's size, live switch and clear, unregistered names ignored, styles on top).
• utils (readerOverrides.test.ts): per-key overlay, word/dark re-derivation, explicit-dark preservation, positionless storage, unknown-style-number and empty-input tolerance, lowercase normalization.

Host-facing documentation added to the @doenet/doenetml-iframe README; the styling concepts page now describes reader remapping as available.

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