fix(plugin-vite): register co-located islands added while dev server runs

[limkaa4]

Problem

Creating a new island inside a (_islands) folder while the dev server is
running does nothing — the island stays unregistered until the server is
restarted. Editing an existing island works fine, so it reads like flaky
HMR rather than a missing registration.

The layout is the documented one from
concepts/file-routing.md:

routes/
├── (marketing)/
│   └── (_islands)/
│       └── interactive-stats.tsx
└── shop/
    └── (_islands)/
        └── cart.tsx

Cause

isIslandPath() in client_snapshot.ts gates what the watcher feeds into
the client snapshot, and its co-located branch is unreachable:

const relIsland = path.relative(options.islandsDir, filePath);
if (!relIsland.startsWith("..")) return true;

const relRoutes = path.relative(options.routeDir, filePath);

if (!relIsland.startsWith("..") && relRoutes.includes("(_islands)")) {
  return true;
}

We only reach the second if when relIsland does start with "..", so
re-testing the same value is always false. relRoutes is computed and
then never used for the decision.

The initial crawl in fs_crawl.ts handles these islands correctly, which is
why a restart fixes it — only files created after startup are affected.

Fix

Test relRoutes, and match (_islands) as a path segment rather than a
substring, matching GROUP_REG in fs_crawl.ts. Files outside both
directories now bail out early instead of relying on a substring check.

Tests

client_snapshot_test.ts covers the top-level islands/ dir, both
documented co-located placements (inside a route group and inside a plain
route folder), and the negative cases — (_components), a (_islands)x
substring, and a matching folder outside both directories.

Verified the tests fail against the current implementation and pass with the
fix. deno fmt, deno lint, deno task check:types and
deno test -A packages/plugin-vite/src/plugins/ are clean.

No integration test: the neighbouring dev-server HMR test is already marked
ignore: true for flakiness, and the regression is fully captured by the
pure function.

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