fix(newos4): never overwrite a directory node; ls honours a path after its flags

MAINT-010 and MAINT-016, the two high items the review left open. Both were re-probed
before the change and the new test is red on HEAD (5 passed, 7 FAILED).

- virtual-fs.js rename()/copy(): they assigned dst.dir.content[dst.name] with no destination
  type check, so `mv file /docs` replaced the /docs node and every child vanished while the
  command printed "Moved: …". Refuse when the destination is a real directory node (the
  virtual /sys entry is exempt). rename() also erased the file on `mv a a`, because assigning
  and then deleting the same key removes it; that path is now an explicit no-op.
- module-fs.js mv/cp: a refusal used to surface as "No such file or directory", which sent
  the operator looking for a missing source. The destination is now inspected first and the
  message says "is a directory — refusing to overwrite it".
- module-fs.js ls: the path was only read from argv[0], so `ls -l /work` listed the cwd.
  The first non-flag argument is now the target; the path-first form is unchanged.

Tests: modules/test/test-fs-mv-cp-guard.js (12 asserts) — RED on HEAD (A1-A3, B1, B2, C2, E1),
GREEN now; maintain --fix reports "node tests 7/7 pass". Browser on v4.6.104:
"mv: /vtest: is a directory — refusing to overwrite it", child and source intact,
"ls -l /vtest" lists /vtest (keep.txt), not the cwd.

1 files changed, 8 insertions, 0 deletions

diff --git a/modules/core/virtual-fs.js b/modules/core/virtual-fs.js
index 3ed1b40..3a3e119 100644
--- a/modules/core/virtual-fs.js
+++ b/modules/core/virtual-fs.js
@@ -686,6 +686,11 @@ class VirtualFileSystem {
         if (!dst || !dst.name) return false;
         const node = src.dir.content[src.name];
         if (!node) return false;
+        // mv a a is a no-op, not a delete (assign-then-delete on the same key used to erase it)
+        if (src.dir === dst.dir && src.name === dst.name) return true;
+        const existing = dst.dir.content[dst.name];
+        // Never clobber a directory node: its children would vanish with it (MAINT-010).
+        if (existing && existing.type === 'dir' && !existing._virtual) return false;
         dst.dir.content[dst.name] = node;
         delete src.dir.content[src.name];
         this.saveToStorage();
@@ -701,6 +706,9 @@ class VirtualFileSystem {
         if (!dst || !dst.name) return false;
         const node = src.dir.content[src.name];
         if (!node) return false;
+        const existing = dst.dir.content[dst.name];
+        // Never clobber a directory node: its children would vanish with it (MAINT-010).
+        if (existing && existing.type === 'dir' && !existing._virtual) return false;
         dst.dir.content[dst.name] = JSON.parse(JSON.stringify(node));
         this.saveToStorage();
         this._emitChange(dstPath);