Fix P1 review issues: race condition, fast-path bypass, git revert, timestamp guard

1. Fix cold-start race in /api/update-check: if the background thread has
   not finished fetching the manifest when the endpoint is hit, do an
   inline fetch so the UI never permanently caches empty defaults.

2. Fix PyPI fast-path blocking manifest git-main installs: evaluate the
   manifest unsloth_source directive BEFORE the PyPI version check so
   users already on latest PyPI still switch to git main when directed.

3. Fix base.txt reverting git install: filter out "unsloth" from base.txt
   requirements when the git-main path is active, preventing pip/uv from
   replacing the git checkout with the PyPI wheel.

4. Guard install timestamp write: only write UNSLOTH_STUDIO_INFO.json
   when python deps were actually updated, preventing users from clearing
   a critical-update warning by re-running setup in llama-only or
   skip-python-deps mode.

studio/install_python_stack.py

@@ -534,13 +534,21 @@ def install_python_stack() -> int:
                 f"git+https://github.com/unslothai/unsloth.git@{_git_ref}",
                 constrain = False,
             )
-            pip_install(
-                "Updating remaining base packages",
-                "--no-cache-dir",
-                "--upgrade-package",
-                "unsloth-zoo",
-                req = REQ_ROOT / "base.txt",
+            # Filter out "unsloth" from base.txt so pip does not
+            # revert the git install back to the PyPI wheel.
+            _base_no_unsloth = _filter_requirements(
+                REQ_ROOT / "base.txt", {"unsloth"}
             )
+            try:
+                pip_install(
+                    "Updating remaining base packages",
+                    "--no-cache-dir",
+                    "--upgrade-package",
+                    "unsloth-zoo",
+                    req = _base_no_unsloth,
+                )
+            finally:
+                _base_no_unsloth.unlink(missing_ok = True)
         else:
             # Update path: upgrade only unsloth + unsloth-zoo while preserving
             # existing torch/CUDA installations.  Torch is pre-installed by