Minify: Use the minifier's decoded source map where available

[robhogan]

When minifying, the transform worker converted Babel's decoded map to tuples and encoded it for the minifier, then decoded the minifier's output map with source-map (toBabelSegments), converted it back to tuples and encoded it again as the stored VLQ map. Terser already builds that decoded map, and only encodes it when result.map is read.

This diff:

• Adds an optional decodedMap to MinifierResult. When a minifier provides it, the worker encodes the stored map straight from it with vlqMapFromBabelDecodedMap, as it already does for unminified modules. Otherwise it decodes map with @jridgewell/sourcemap-codec, which is already installed via @babel/generator.
• Has metro-minify-terser return Terser's decoded_map as decodedMap, and encode map only if it's read.
• Passes Babel's own encoded result.map to the minifier, rather than re-encoding it from tuples.
• Deprecates MinifierResult.map in favour of decodedMap, and toBabelSegments, toSegmentTuple and tuplesFromBabelDecodedMap in metro-source-map, which Metro no longer uses. The next diff removes them.

Output is unchanged - across every module of our benchmark app*, the minified code, line count and stored map are byte-identical. Custom minifiers that only return map keep working as before.

Minifying the whole app is 10.4% faster (95% CI 7.4-11.4%; 21.3s -> 19.2s of CPU on our benchmark app*), from no longer converting the map to and from tuples, and from Terser no longer encoding its map at all:

	Minify step (median)	vs main
main	21,286ms (20,844 to 21,561)
This diff	19,228ms (18,886 to 19,773)	-10.4% (-11.4 to -7.4)

Changelog

 - **[Performance]**: Faster source map handling when minifying, using the minifier's decoded map if it provides one (`MinifierResult.decodedMap`)
 - **[Deprecated]**: Custom minifiers should return their source map as `decodedMap` - `MinifierResult.map` is deprecated
 - **[Deprecated]**: `metro-source-map`: Deprecate `toBabelSegments`, `toSegmentTuple` and `tuplesFromBabelDecodedMap`

Test plan

New tests cover the worker using decodedMap without reading map, and metro-minify-terser returning Terser's decoded map and encoding map only on request.

yarn jest
yarn flow check
yarn lint
yarn verify-api-snapshots

* Benchmark: Mattermost Mobile 2.45.0 (React Native 0.83.9), its release bundle split back into its 7,082 modules (5.13M mapping segments), each minified with metro-minify-terser from its code and map as the worker does. Times are single-threaded totals across all modules - medians of 15 rounds interleaving main and both diffs in this stack, Node 22.13.1 on an M5 Pro, with 95% bootstrap confidence intervals. Output was checked byte-identical in every run.