feat: routeTransforms option for non-TS route languages

[STRd6]

Fixes #1549 (on main/v2; the vinxi-based 1.x line has the same issue — happy to backport, see notes below).

Problem

Route modules are analyzed for their exports by reading the file off disk and parsing it with esbuild's tsx loader (analyzeModule in fs-routes/router.ts, vendored from vinxi). Languages that compile to JS/TS — Civet, CoffeeScript, etc. — can't be parsed that way, so toRoute throws, addRoute swallows the error, and the route is silently dropped even though the user registered the extension via extensions and added a bundler plugin that handles the language everywhere else.

Reported here as #1549 and to Civet as DanielXMoore/Civet#1319. Civet currently works around it by monkey-patching toRoute and substituting fs.readFileSync during analysis (DanielXMoore/Civet#2148) — this PR is the seam that makes that workaround unnecessary.

Approach

A per-extension source transform that runs before export analysis, so all the existing pick/route-config/HTTP-method logic is reused and the transform author never sees es-module-lexer's shapes:

import civet from "@danielx/civet";

solidStart({
  extensions: ["civet"],
  routeTransforms: {
    civet: (code, sourcePath) => civet.compile(code, { filename: sourcePath, sync: true }),
  },
})

• FileSystemRouterConfig gains routeTransforms?: Record<string, RouteModuleTransform> (extension without dot, matching how extensions is spelled), threaded through solidStart().
• analyzeModule(src) is split into analyzeCode(code, sourcePath) plus a new analyzeRouteModule(sourcePath, config) that applies the matching transform first (and await inits the lexer, which watcher-triggered updates could otherwise race).
• toRoute becomes async; its only callers (addRoute / updateRoute) were already async and now await it.

The hardcoded .md/.mdx carve-out in toRoute is untouched, but could conceivably migrate to this mechanism later.

Verification

tsc --noEmit on packages/start is clean. vitest run shows 2 failures that also fail for me on unmodified main (server-fns related), 5 passing.

Notes

• Naming (routeTransforms, sourcePath) and shape are open to feedback — e.g. this could alternatively hang off extensions entries in a tuple form like vite-plugin-solid's extensions option.
• The released 1.x line hits the same problem through vinxi's analyzeModule; since toRoute lives in this repo there too, a near-identical backport is possible without touching vinxi.

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