perf: speed up client startup

[afonsojramos]

Summary

Cold startup does much less waiting before modules run.

• Webpack capture. The capture registers its chunk and then always slept 500 ms before checking the result. The runtime runs the chunk callback inside push, so the capture now returns as soon as it succeeds.
• Scrolling fix. The workaround is meant for Spotify 1.2.56 and older, but its check (version[1] >= 2 && version[2] >= 57) let it run on 1.3.0 to 1.3.56. On every DOM mutation it called getComputedStyle on every element, about 400 ms of main-thread time per boot on 1.3.3, and it kept forcing compositor layers on scrollable elements. A component-wise isVersionBefore helper now limits it to the versions it was written for.
• Theme first. The boot theme and its dependencies load before other modules, still in dependency order, so the client renders already themed. Theme stylesheets are kept after every other module stylesheet, so themes still override extension styles at equal specificity whatever order modules load in.
• Color schemes only for themes. Every module with CSS used to fetch /modules/<id>/color.ini before its load, one module at a time (about 35 ms each). Only themes ship a color.ini, so extensions no longer look for one. That removes about 450 ms of serial requests with 12 styled extensions.
• Prefetch. After mixins, the loader adds modulepreload for each staged module's JS and starts fetching its CSS, using the same skip rules as runLoads. Apply now records every other script of a multi-file module in the manifest (preload, skipping tests, specs and tool configs), so a tree like stdlib's 119 files arrives in one batch instead of five or six dependent import waves. Nothing is evaluated early. Unused prefetched CSS is dropped after boot, and a failed CSS prefetch is logged before the loader fetches again.
• waitForClient polls every 25 ms instead of 200 ms.

Results

Spotify 1.3.3.264, macOS arm64, 19 installed modules with the Text theme. These are 12 interleaved cold renderer boots per setup, measured over CDP with the machine under heavy unrelated load (load average about 8). Median, with the 10th to 90th percentile in brackets:

Milestone	Released CLI + stdlib 1.13.1	This PR + spicetify/modules#48
Theme applied	2206 ms [2058–2327]	572 ms [520–615]
All modules loaded	2297 ms [2273–2378]	1412 ms [1328–1624]
First home content	849 ms [814–971]	978 ms [894–1118]

First home content arrives about 130 ms later because the home page now renders with the theme already applied. Before, it rendered unthemed and then restyled when the theme arrived about 2.2 s in, so the themed home now appears more than a second earlier. An A/B with and without the preload list, 12 boots each, showed the list is not the cause: home content came at 931 ms with it and 985 ms without, and all modules loaded at 1458 ms against 1511 ms.

Validation

• All 191 src/jsHelper tests and all 166 spicetify library tests pass (4 ignored, as on v3-beta). cargo clippy -p spicetify --release -- -D warnings is clean.
• New tests cover:
  • synchronous capture and the version helper
  • theme-first ordering and scheme lookup only for themes
  • prefetch selection: disabled and blocked dependencies, other themes, mixin modules, local installs and preload lists
  • stylesheet ordering against happy-dom's adoptedStyleSheets
  • the manifest script list
• Live, on a client applied with this branch: the scrolling fix no longer tags elements on 1.3.3, every module entry and all of stdlib's scripts are fetched within the first 70 ms, and no extension requests a color.ini.

Not verified: Windows and Linux, and the Go CLI's use of the shared wrapper on Spotify versions before 1.2.57.

A related bug is left for a separate change. The CosmosAsync proxy in platform.js uses the same broken check (version[1] >= 2 && version[2] < 31), so on 1.3.0 to 1.3.30 it returns Spotify's unwrapped fetch. Fixing it changes request handling for every module on 1.3, so it needs its own testing.

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