fix: empty directories return ENOENT under libuv (missing "." / ".." in ReadDirectory)

[hooyao]

Symptom

VS Code (and any libuv / Node.js / Electron app) on a RamDrive-backed TEMP fails to enumerate empty directories. VS Codes auto-update aborted every time with:

ENOENT: no such file or directory, scandir Z:\Temp\vscode-stable-system-x64
    at async readdir (node:internal/fs/promises)
    at async pa.cleanup (.../main.js)

VS Codes cachePath getter does mkdir(cacheDir,{recursive:true}) and immediately readdir(cacheDir). On a fresh RAM-disk TEMP the cache dir is newly-created and empty -> readdir -> ENOENT -> the update never proceeds (repeats identically on every retry).

Root cause

WinFspRamAdapter.ReadDirectory enumerated only real children -- it never emitted the synthetic . (self) and .. (parent) entries that NTFS and WinFsps reference memfs produce for every non-root directory.

An empty directory therefore enumerated to zero records. The Windows kernel answers the first NtQueryDirectoryFile on a zero-record enumeration with STATUS_NO_SUCH_FILE (-> ERROR_FILE_NOT_FOUND):

• .NET Directory.* / PowerShell Get-ChildItem swallow that status and return an empty set -- so the bug was invisible to the existing .NET-based torture/chaos suites and to manual Get-ChildItem.
• libuv (Node.js / Electron) maps it straight to ENOENT -- so VS Code, npm, git, etc. break.

Reproduction (100% deterministic, against the live mount)

case	result
mkdir(d); readdir(d) on Z: (empty)	ENOENT
same dir after dropping one file into it	OK (flips on non-empty)
empty dir on C: NTFS (control)	OK []

mkdir; sleep 50ms; readdir does not help -- ruling out the kernel-cache / async-Notify race the cache-coherency postmortems primed us to suspect. The trigger is purely "directory enumerates to zero records".

Fix

Mirror the validated MemfsReference oracle: prepend . and .. for non-root directories in ReadDirectory, with correct marker-based resume (./.. sort first; children listed fresh afterwards). Root is excluded -- NTFS root has no ./...

Test

tests/RamDrive.IntegrationTests/DirectoryEnumerationTests.cs -- must go through raw Win32 FindFirstFileW, the only layer that exposes the bug (Directory.* passes even when broken).

• Verified fails 4/4 without the fix (FindFirstFile -> ERROR_FILE_NOT_FOUND) and passes 4/4 with it (deliberate-break self-test).
• Full suite this branch: unit 36/36, integration 36 pass.

Note: TortureTests.OverwriteTruncateExtend fails on this dev machine, but it also fails on main without this change (it exercises file overwrite, not directory enumeration) -- pre-existing and out of scope here.

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