Fix fatal panic on non-absolute path in VFS

[Copilot]

The VFS layer panics when a non-absolute path reaches RootLength() via the module resolution pipeline. This can occur when containingDirectory is empty, causing normalizePathForCJSResolution("", "./foo") to produce a relative path that propagates through FileExists → Stat → RootAndPath → SplitPath → RootLength.

TypeScript's sys.fileExists simply returns false for such paths. The Go port's defensive panic is too strict.

Changes

• RootLength returns 0 instead of panicking for non-absolute paths
• SplitPath returns ("", "") when root length is zero
• RootAndPath returns nil fsys when rootName is empty, causing downstream FileExists/DirectoryExists to return false and ReadFile to return ("", false)
• Updated test that asserted panic behavior to assert graceful false/empty return
• Added a focused module resolver regression test for resolving a relative specifier with an empty containing file

[DanielRosenwasser]

@copilot you did not add a test

[Copilot]

@copilot you did not add a test

Added a focused regression test in internal/module/resolver_test.go covering ResolveModuleName("./foo", "", ...), which previously reached the non-absolute VFS lookup path and panicked. Addressed in f7f88aa.

[DanielRosenwasser]

@copilot can you investigate adding a real world example as a compiler or fourslash test that fails without any fix?

[Copilot]

@copilot can you investigate adding a real world example as a compiler or fourslash test that fails without any fix?

I investigated compiler and fourslash coverage paths. Standard compiler tests normalize @filename units against an absolute current directory, and fourslash normalizes fixture files under / or ^/ for dynamic URIs, so the usual real-world import/untitled-file cases do not reproduce the pre-fix empty-containing-file panic. I also tried a relative CLI root-file repro; that path is normalized before module resolution as well. Because the failing condition is specifically a resolver/VFS call after file context has already become empty/relative, I did not add a compiler/fourslash test that would give false confidence; the focused resolver regression remains the only test I found that fails without the fix.