fix(winpython): narrow search paths and add discovery cache (#456)

Fixes #453.

The WinPython locator was scanning `C:\`, `D:\`, `E:\`, `Program Files`,
`~/Downloads`, `~/Desktop`, and `~/Documents` on every refresh — all
Windows Defender hot-spots — to find a niche portable distribution. That
inflated p90 refresh latency for every Windows user, regardless of
whether WinPython was installed.

## Changes

- Restrict default search to `%USERPROFILE%\WinPython`.
- Add opt-in `WINPYTHON_HOME` env var (single path or `;`-separated
list; each entry can *be* or *contain* a WinPython install).
- Add a per-process discovery cache
(`Arc<Mutex<Option<Arc<Vec<...>>>>>`), mirroring `WindowsStore` and
`WindowsRegistry`, with `SyncedDiscoveryState` refresh persistence.
- Extract `build_search_paths(userprofile, winpython_home)` as a pure
helper so tests don't have to mutate process env.
- New regression tests pinning the search-path policy and
`WINPYTHON_HOME` parsing on both Windows and Unix.
- README: document `WINPYTHON_HOME` and the new search-path policy.

`try_from()` behavior on a real WinPython executable is unchanged — it
still walks parent directories looking for marker files / `WPy*`
directory names.

Author: karthiknadig

.gitignore

@@ -18,4 +18,4 @@ tmp/
 temp/
 docs/node_modules/
 docs/package.json
-docs/package-lock.json
+docs/package-lock.jsonpr-status.json

crates/pet-winpython/README.md

@@ -30,3 +30,26 @@ WPy64-31300/                          # Top-level WinPython directory
 ## Platform Support
 
 This locator only works on Windows, as WinPython is a Windows-only distribution.
+
+## Search Paths
+
+By default, the locator only looks for WinPython installations under
+`%USERPROFILE%\WinPython`. Earlier versions also scanned drive roots
+(`C:\`, `D:\`, `E:\`), `Program Files`, `Downloads`, `Desktop`, and
+`Documents` on every refresh — those scans were a Windows Defender
+hot-spot and inflated refresh latency, so they have been removed.
+
+If your WinPython installation lives elsewhere, point the locator at it
+with the `WINPYTHON_HOME` environment variable. The value can be a
+single path or a `;`-separated list of paths. Each entry can either:
+
+- _be_ a WinPython installation (e.g. `D:\WPy64-31300`), or
+- _contain_ one or more WinPython installations (e.g. `D:\python-tools`
+  with `D:\python-tools\WPy64-31300` inside).
+
+Examples:
+
+```
+set WINPYTHON_HOME=D:\WPy64-31300
+set WINPYTHON_HOME=D:\WPy64-31300;E:\portable-python
+```