ThemeProvider: stop bundling the legacy JS theme with the root entry #8722

Part of #8676 (P2). Breaking change. This needs a decision on major-version timing before anyone starts.

Problem

ThemeProvider.tsx:5 imports theme.ts, which imports legacy-theme/ts/color-schemes.ts (327 KB of source). index.ts:3 also exports theme. Every consumer of the root entry therefore ships and evaluates the legacy theme.

Measured: with esbuild on theme.ts alone:

• 275 KB minified.
• 20 KB gzipped. The gzipped size is small because the data repeats, but parse and evaluation cost follows the 275 KB.

The per-provider deepmerge(theme, scheme) is cheap: 0.4 to 0.6 ms warm and 1.5 to 3 ms on the first call.

@primer/react/next already avoids the theme import, and the root ThemeProvider is deprecated in favor of it.

Proposed approach

• Decide which major release removes the default theme from the root ThemeProvider and the root theme export.
• Ship a migration path, either @primer/react/next or an explicit theme prop import.
• Consider shipping this alongside ThemeProvider: Deprecate redundant day and night color modes #3246 (deprecate the day and night color modes), another breaking ThemeProvider change for the next major.

Acceptance criteria

• The target major release is decided and recorded here.
• Importing the root entry no longer pulls in legacy-theme/ts/color-schemes.ts. Verify with a bundle analysis.
• Migration guidance is published for consumers that rely on the default theme or the theme export.