Closes #369.

Themes now own their icon glyphs, not just their colours. The standalone icons prop is removed; a theme supplies glyphs via Theme.icons (keyed add/edit/delete/copy/ok/cancel/collection), each an IconDefinition (content plus optional viewBox/svgProps/scale). User-supplied glyphs are themeable through currentColor exactly like the built-ins, and per-instance overrides compose through the same theme layering as style overrides — one mechanism instead of two.

Core (json-edit-react)

• New IconDefinition / ThemeIcons types; Theme gains icons?. IconReplacements and JsonEditorProps.icons removed.
• Built-in glyphs are authored as IconDefinitions on defaultTheme.icons (each carrying its scale correction); defaultTheme.ts → .tsx.
• mergeIcons composes icons across the theme array (defaultTheme as layer 0), so the merged set is always complete — the renderer drops its per-name switch/fallback and derives the paint key by the naming invariant (icon + PascalCase).
• The seven IconAdd…IconChevron components + IconProps are gone (their markup now lives in defaultTheme.icons). IconSvg is exported as the glyph renderer — pass an IconDefinition's parts; it owns the size baseline so the ratio constant stays internal.
• The chevron glyph key is renamed collection; the collapse chevron stays hover-inert (no jer-icon).

@json-edit-react/utils

• New iconFromSvg(svg): builds an IconDefinition from a raw SVG string (full <svg>…</svg> or bare inner markup), a React <svg> element (unwrapped via its props/children), or an existing IconDefinition (passthrough). String inputs are interned for inline referential stability — reuses a generalised intern promoted to a shared module (filters behaviour unchanged).

@json-edit-react/themes

• Build readiness for themes that ship icons: react/react/jsx-runtime are kept external (never bundled), and react is declared an optional peer dep. No current theme ships glyphs yet.

Demo

• The "Edit this theme!" dataset renders each IconDefinition as the actual themed glyph via the exported IconSvg (display-only) instead of the raw React-element internals.

Docs

• README: documents Theme.icons / IconDefinition / the currentColor rule / scale; drops the icons prop row; updates the exports list.
• Migration guide: new §15 — icons prop → theme.icons, JSX.Element → IconDefinition/iconFromSvg, chevron → collection, and the removed exports.
• Changesets: core (major), utils (minor), themes (minor).

Breaking changes

• icons prop removed → use theme.icons.
• IconAdd…IconChevron, IconProps, IconReplacements no longer exported.
• chevron icon key → collection.

Testing

• New test/icons.test.tsx (core) and packages/utils/test/icon.test.tsx. Full suites green (643 core tests), lint clean, demo typechecks.

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