wp-build-polyfills: strip minified sourcemaps from production builds

[louwie17]

Not sure if we will go ahead with this yet, as it would mean that SCRIPT_DEBUG=true is of no use

Proposed changes

@wordpress/build (wp-build) hardcodes sourcemap: true for package bundles, so every production build emits *.min.js.map files under build/modules/** and build/scripts/**. Since the whole build/** tree is production-include, those maps ship to the package mirror and into consumer plugin zips — ~11 MiB for Premium Analytics today (e.g. build/modules/ui/index.min.js.map alone is 3.2 MiB). Widget and route bundles get no sourcemaps at all, so the maps that do ship are inconsistent dead weight. (No upstream toggle exists yet; nearest analogs are WordPress/gutenberg#75395 and WordPress/gutenberg#75993.)

• Extend strip-unminified-prod in wp-build-polyfills to also delete *.min.js.map / *.min.css.map under its target subdirs (routes, scripts, modules, styles, widgets).
• Strip the trailing sourceMappingURL comment from each minified asset whose map was deleted, so browsers don't request a 404'd map.
• Report the removed sourcemap count in the bin's summary line and in strip()'s return value (deletedMaps).
• Extend the end-to-end test (real wp-build output) to assert maps are deleted and asset references stripped, and that the pass stays idempotent.

Related product discussion/links

• Upstream: wp-build: Add --no-script-debug flag to skip unminified asset generation WordPress/gutenberg#75395 (proposed --no-script-debug flag), Build: Skip sourcemaps for WASM-inlined script module workers WordPress/gutenberg#75993 (sourcemaps disabled for WASM workers for the same size reason)

Does this pull request change what data or activity we track or use?

No.

Testing instructions

• cd projects/packages/wp-build-polyfills && pnpm run build && pnpm run test — all tests pass, including strip-unminified-prod-real-fixture.test.js which builds a real wp-build fixture and asserts the .min.js.map files are removed and their sourceMappingURL references stripped.
• For an end-to-end check: cd projects/packages/premium-analytics && pnpm run build-production, then confirm find build -name '*.map' returns nothing and grep -r sourceMappingURL build --include='*.min.js' finds no references (the summary line reports removed ... 8 minified sourcemap(s)).
• Confirm the dashboard still loads normally from such a build (loaders collapse to .min.js as before; only the map files and their references are gone).

[github-actions]

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Code Coverage Summary

Coverage changed in 1 file.

File	Coverage	Δ%	Δ Uncovered
projects/packages/wp-build-polyfills/bin/strip-unminified-prod-lib.js	251/266 (94.36%)	1.18%	0 💚

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