fix(store): read the core version from disk, not from a stale import (#518)

* fix(store): read the core version from disk, not from a stale import

Updating the core to 3.3.0 and then updating plugins refused all eight sports
scoreboards:

    Refusing to install nrl-scoreboard: NRL Scoreboard supports LEDMatrix
    >=3.3.0, but this system is running 3.2.0.

while src/__init__.py on that machine read 3.3.0. Observed on hardware, not
theorised.

The gate ran `from src import __version__ as core_version`, which binds
whatever the process loaded at start. The plugin store's gate lives in the web
UI, a long-lived service of its own, and the update route deliberately restarts
nothing -- it replaces files on disk and asks the user to restart. Its prompt
named only the *display* service, so a user who followed it left the web
process holding the previous number.

Stale by exactly one release is the case that bites: every plugin flooring on
the release you just installed is refused, blaming a core version that is
already correct on disk. It reads as a broken plugin store. 3.3.0 is the first
release where this hits a whole family at once, since all eight scoreboards
floor there.

compatibility.current_core_version() reads the version from the file instead,
falling back to the imported value on any failure -- so it can only ever be as
correct as before, never worse. All four gate call sites use it: three in
store_manager (install, the git-pull update path, install_from_url) and one in
plugin_loader's advisory warning.

The restart prompt now names both services.

Twelve tests, including the hardware failure itself: a process holding 3.2.0
while disk says 3.3.0 refuses hockey, and reading fresh allows it. The inverse
is asserted too -- a genuinely old core still refuses, so the gate has not
become permissive. One test greps both modules for the old import-bound read;
reintroducing that line fails it, which is what stops this coming back.

Not changed: web_interface/__init__.py also imports __version__, but for
display rather than gating, and the API endpoint already reports a fresh
git describe.

* fix: drop the unused os import

Left over from a first draft that joined paths by hand before this used
pathlib. Flagged by CodeRabbit on #518; confirmed dead -- no os. reference
remains in the module.

src/plugin_system/plugin_loader.py

@@ -772,8 +772,8 @@ class PluginLoader:
         newer than the running core. Advisory only — never raises — so a
         plugin that guards optional features with try/except keeps working.
         """
-        from src import __version__ as core_version
         from src.plugin_system import compatibility
+        core_version = compatibility.current_core_version()
 
         compatible, _reason = compatibility.check(manifest, core_version)
         if compatible: