fix(cli): resolve the Bun preload path against the package root (#12387)

Under Bun the server child is spawned with --preload <path>/open-sse/utils/setupPolyfill.ts, and all three spawn sites built that path next to the server bundle — but the polyfill only ships at the package root and nothing copies it into dist/. Every `bun install -g omniroute` start died with `error: preload not found`. The preload now resolves from the supervisor module's own location and is shared by the two serve.mjs spawns, with the child argv moved into a pure buildServerSpawnArgs() so both branches are directly assertable (same seam as #8131). Node users are unaffected.

Validated in a combined worktree with all 25 PRs of this batch boarded together: typecheck:core clean, 443/443 node-runner tests plus 14/14 vitest across every test file the batch touches, and check-changelog-integrity, check:cycles (418 files), check:provider-consistency (272 REGISTRY entries, 355 canonical providers), check:docs-counts, check:docs-sync (42 locales) and check-file-size all green.

Thanks @pacocartones.