fix(hmr): allow watch-only mode without loader internals

[XuJianrui]

Problem

HMR crashes on mount in watch-only mode (root: []) when the loader does not expose its internal module loader, and the error message misleadingly points at --expose-internals — which that mode does not need.

Root cause

loader.internal is resolved by fromInternal() based on whether Node was launched with --expose-internals, and may be undefined. The Hmr constructor previously required it unconditionally, violating the ModuleLoader | undefined contract the loader declares.

Changes

• The constructor only asserts loader.internal when root.length > 0 (module-reload mode); watch-only mode creates no watcher and tracks no externals.
• The internal field is now typed ModuleLoader | undefined. Module-reload paths use a requiredInternal accessor (fails fast with an actionable error if somehow missing); getLinked degrades via ?. since it is reachable from external callers; the change handler also uses ?. as a second layer of defense (the watcher itself is gated by root.length); partialReload guards with a warning instead of crashing.
• _resolve now throws on an unrecognized loader internal version instead of silently returning undefined; the LOADER_INTERNALS_ERROR message is unified and tells the user both ways to resolve it (--expose-internals, or watch-only root: []).
• Test directory resolution switches to import.meta.dirname, fixing the double-drive bug of .pathname on Windows; isBuiltinOrExternal is extracted to remove a duplicated predicate.

Tests

3 new regression cases run on every CI (the repo's vitest.config.ts sets --expose-internals); the t.skip() in the watch-only × internal-present case is a defensive fallback for local runs without the flag. Module-reload × present is already exercised by the existing reload tests. Package tests pass 29/29.

Why

• loader is left untouched: it already guards internal correctly (e.g. config/tree.ts checks if (this.ctx.loader.internal)), so editing it would only broaden the fix's scope.
• The three internal access styles are intentional: requiredInternal fails fast on the module-reload path; ?. degrades silently for the public getLinked and acts as a second layer of defense in the change handler (the watcher itself is gated by root.length); partialReload warns instead of crashing.
• The import.meta.dirname switch and the isBuiltinOrExternal / LOADER_INTERNALS_ERROR extractions are behavior-preserving cleanups kept with the fix rather than split out, since they are low-risk and already covered by the new tests.