feat(metro): surface compiler warnings during a build

[YevheniiKotyrlo]

Problem

Closes #424.

On main, compile(css).warnings() collects everything the compiler could not translate, but src/metro/metro-transformer.ts calls .stylesheet() and discards the rest of the compile result. So a dropped float or z-index: auto is silent in every build.

lightningcss also warns about input it cannot parse, such as an unknown at-rule or an unrecognised pseudo-element. lightningcss passes the rule through, the visitor finds nothing in it, and the rule disappears. The compiler never reads those warnings.

Solution

• The transformer hands .warnings() to reportCompilerWarnings, which prints one block per stylesheet:

  react-native-css: src/global.css - 3 declarations dropped, no React Native equivalent
    properties: columns, float
    values: z-index: auto
  

  A file's block prints when its set of warnings changes, so an incremental rebuild that changes nothing about them stays quiet. Warnings never fail a build.

• A warnings option on withReactNativeCSS sets how much is printed. Issue compile().warnings() is unreachable from a real build: 50 addWarning call sites, no Metro consumer #424 offered three shapes, and this PR implements option 1 with option 2's escape hatch:

  • "summary" (the default) caps each channel at ten entries;
  • "verbose" lists everything;
  • "none" prints nothing.

  An opt-in default would leave the channel unseen by anyone who doesn't read the docs. If you'd rather it defaulted to "none", that's a one-word change.

• lightningcss's parse warnings from the first pass go to a new syntax channel. It is kept apart from what React Native cannot express, because the fix for it is in the author's stylesheet. This package's own @nativeMapping is not reported.

• The README documents the option.

Tests

• src/__tests__/metro/warnings.test.ts builds its fixtures from real compile() results and covers:
  • the formatting;
  • the caps;
  • the syntax clause;
  • re-printing only on a change;
  • a file that stops warning and warns again;
  • paths inside and outside the project root.
• src/__tests__/metro/metro-transformer.test.ts drives the transformer with a stand-in for Expo's transform worker, and checks that the level and the file reach the report. It also checks that web and non-CSS files never print.
• src/__tests__/compiler/warnings.test.ts covers the syntax channel: an unknown at-rule and an unrecognised pseudo-element, each dropping its rule; a correctly spelled control; this package's own at-rule; and a repeated mistake.
• On main, 6 of the 14 cases in the files that load there fail; warnings.test.ts exercises the new module. Each of 17 mutations fails its own cases.
• yarn example expo export --platform android prints the block for the example's global.css: 42 declarations from Tailwind's preflight.

Verification

On Windows with Node 26:

• yarn lint clean
• yarn typecheck clean
• yarn test --coverage: every failure also fails on main on this machine (the four babel cases)
• yarn build clean
• yarn example expo export --platform web exported

Merge order

It shares lines with #411 (README.md), #460 (compiler/compiler.ts); whichever lands second rebases.

It also shares lines with #428 (README.md); whichever lands second rebases.

Base

Re-written on main (a5002c5), and no longer stacked on #390: nothing here depends on its babel changes. main refuses @react-native blocks outright, so the filter for this package's own at-rules now covers only @nativeMapping.

[YevheniiKotyrlo]

No device frame

The change is terminal output, and the description shows the block a build prints.