feat(loader): add the disk cache option

[dangreen]

Adds a cache boolean to @srcset/loader, mirroring the option of @srcset/vite-plugin but off by default.

The asymmetry is deliberate. The vite plugin has to cache: Vite has no persistent cache for plugin work, so without it every dev server start regenerates every variant. Webpack and rspack do have a persistent cache, so when it is enabled it already covers the loader - measured on the rspack example, 30 s of image generation on a cold start drops to 2 s on the next one, and the dev server comes up in 154 ms.

But that cache is off by default in both bundlers, and a project that does not enable it pays the full generation cost on every start. cache: true gives the loader its own storage under node_modules/.cache/srcset, independent of the bundler settings - on the same example, 30 s to 1 s. Keyed by the source contents and the generate options, so it also survives config edits that invalidate the bundler cache wholesale.

• SrcSetLoaderOptions takes cache?: boolean and drops the inherited SrcSetCacheStorage field, the same shape SrcSetVitePluginOptions uses
• getSharedCache(context) memoizes one storage per compiler context, mirroring getSharedLimit
• Only the module options reach generateSrcSetModule now - the loader-specific ones (name, emitFile, publicPath, …) no longer leak into it
• Covered on both webpack and rspack: with the option a second build does not run the optimizer again and still emits the assets; without it the optimizer runs twice

🤖 Generated with Claude Code

[coveralls]

Coverage Report for CI Build 32296723641

Coverage decreased (-0.3%) to 91.37%

Details

• Coverage decreased (-0.3%) from the base build.
• Patch coverage: 2 uncovered changes across 1 file (5 of 7 lines covered, 71.43%).
• 1 coverage regression across 1 file.

Uncovered Changes

File	Changed	Covered	%
packages/loader/src/cache.ts	6	4	66.67%
Total (2 files)	7	5	71.43%

Coverage Regressions

1 previously-covered line in 1 file lost coverage.

File	Lines Losing Coverage	Coverage
packages/loader/src/template.ts	1	95.83%

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Coverage Stats

Relevant Lines:	749
Covered Lines:	700
Line Coverage:	93.46%
Relevant Branches:	572
Covered Branches:	507
Branch Coverage:	88.64%
Branches in Coverage %:	Yes
Coverage Strength:	94291.23 hits per line

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💛 - Coveralls