perf: cache resolved kit files settings in incremental manifest

[inq]

loadKitFilesSettings does a dynamic import() of svelte.config.js on every --incremental run (~137 ms in my profile) just to read 4 paths under kit.files. This caches the resolved settings on the manifest, keyed by mtime+size. Manifest version bumped 3→4 to invalidate older caches.

Measured on a SvelteKit app, warm median: 2.13 s → 2.05 s (3 samples each). Loosely related to #1538.

[changeset-bot]

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Latest commit: dc2903f

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[jasonlyu123]

I am not sure this makes sense. As long as any svelte files are updated, the config still needs to be loaded. The svelte.config.js also has configs used by the Svelte compiler. Caching it isn't as useful as you might think.

[inq]

Hello! Thank you very much and Sorry for my bad English!

But In these cases, I think it can be meaningful enhancements:

1. --incremental + no change -> index.ts#L370 only instantiate SvelteCheck (and configLoader) when filesNeedingDiagnostics.length > 0. So when nothing change, my loadKitFilesSettings is only code path that load svelte.config.js. This case happen a lot in monorepo pnpm typecheck — many packages have no change since last run.

2. .svelte changed -> surprisingly it is slightly faster also! (median of 7 samples each)

version	warm, no change	warm, 1 file touched
svelte-check@4.4.6	2.15 s	2.11 s
this PR	2.05 s	1.99 s

3. About compiler config — configLoader and loadKitFilesSettings consume different parts of the config. We only cache kit.files paths, not preprocess/compilerOptions, so compiler always get fresh config.

4. Savings scale with config size — svelte-check@4.4.6 re-import full dep graph every run. With heavier config (added typescript, crypto, zlib, stream):

version	warm, no change (heavy config)
svelte-check@4.4.6	2.21 s
this PR	2.04 s

So I think it would be positive in every case! But if you think the scope is too small, you can close this!

Thank you very much!

[jasonlyu123]

One problem is that if the files config comes from an environment variable, the cache invalidation won't work. This can happen if people use the files config to have two sets of routes.