composeSourceMaps: Keep mappings that share a generated column

[robhogan]

Context

composeSourceMaps iterates the mappings of the last map, and looked each one's generated position up again in that same map before tracing it back through the earlier maps:

metro/packages/metro-source-map/src/composeSourceMaps.js

Lines 115 to 125 in b7c2055

	for (const consumer of consumers) {
	if (currentLine == null || currentColumn == null) {
	return {line: null, column: null, source: null, name: null};
	}
	original = consumer.originalPositionFor({
	line: currentLine,
	column: currentColumn,
	});
	currentLine = original.line;
	currentColumn = original.column;

That's redundant, except where the last map has more than one mapping at the same generated column, which hermesc emits (15.7k in our benchmark's Hermes map). There, every mapping at the column resolved to the last one and the duplicates were collapsed.

This change

This traces each mapping from its own original position, so the composed map keeps all of them. Lookups through Metro's Consumer are unchanged, since it resolves a column to its last mapping. Consumers that take the first mapping at a column, like @jridgewell/trace-mapping, now get that mapping's own position.

composeSourceMaps is 8.1% faster than #1980 (95% CI 7.6-8.7%) and 2.59x faster than main (878ms -> 805ms on our benchmark app). The output gains 13,867 mappings (+0.8%) at repeated columns and is otherwise identical.

Benchmark

AI-driven.

	Time (median)	vs previous	vs main	Composition memory	vs previous	vs main	Peak RSS
main	2,088ms (2,082 to 2,094)			1,070MB (1,070 to 1,070)			1,664MB
#1980	878ms (872 to 883)		-58.0% (-58.1 to -57.8)	908MB (908 to 909)		-15.1% (-15.2 to -15.0)	1,502MB
This PR	805ms (802 to 811)	-8.1% (-8.7 to -7.6)	-61.4% (-61.6 to -61.1)	911MB (910 to 912)	+0.2% (+0.1 to +0.4)	-14.9% (-15.0 to -14.7)	1,505MB

Changelog

 - **[Fix]**: `composeSourceMaps` keeps every mapping at a generated column that the last map maps more than once

Test plan

New test fails on #1980 (only the last position survives) and passes here.

Benchmark (AI-driven): Mattermost Mobile 2.45.0 (React Native 0.83.9, Metro 0.83.7), iOS release bundle, unminified: 7,086 sources, 52.6MB bundle, 82.4MB flat source map. Composed with its hermesc -O -output-source-map map (hermes-compiler 0.14.1, 10.7MB, 1.84M segments). Timings are composeSourceMaps alone, excluding JSON parsing, over 105 rounds on Node 22.13.1 on an M5 Pro. Each round runs main, every PR in this stack and a memory baseline, in a shuffled order. Figures are medians with 95% bootstrap confidence intervals, for changes of the per-round paired ratio. Composition memory is peak RSS less that of the same process parsing both maps without composing them (594MB).