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/compatibility.py

@@ -28,6 +28,7 @@ fixes their version string. See `docs/SPORTS_UNIFICATION.md`, phase B4.
 from __future__ import annotations
 
 import re
+from pathlib import Path
 from typing import Any, Dict, Optional, Tuple
 
 # Below this, the core's self-reported version is not evidence of anything.
@@ -35,6 +36,45 @@ from typing import Any, Dict, Optional, Tuple
 TRUSTWORTHY_FLOOR: Tuple[int, int, int] = (2, 0, 0)
 
 
+# ``src/__init__.py`` relative to this file: src/plugin_system/ -> src/
+_VERSION_FILE = Path(__file__).resolve().parent.parent / "__init__.py"
+_VERSION_RE = re.compile(r'^__version__\s*=\s*["\']([^"\']+)["\']', re.M)
+
+
+def current_core_version() -> str:
+    """The core version as it is on disk right now, not as it was at import.
+
+    ``from src import __version__`` binds whatever the process loaded at start.
+    The web UI runs as its own long-lived service (``ledmatrix-web.service``),
+    and updating the core replaces files on disk without restarting it -- the
+    update route says so explicitly and asks the user to restart. Its prompt
+    names the *display* service, so a user who follows it leaves the web
+    process holding the old number.
+
+    The plugin store's gate lives in that web process. Stale by one release is
+    exactly the case that matters: every plugin flooring on the release you
+    just installed gets refused, with a message blaming a core version that is
+    already correct on disk. 3.3.0 is the first release where that hits a whole
+    plugin family at once -- all eight sports scoreboards floor there.
+
+    Reading the file costs one stat and a small read per call, and only on the
+    install/update path. Any failure falls back to the imported value, so this
+    can only ever be as wrong as before, never worse.
+    """
+    try:
+        text = _VERSION_FILE.read_text(encoding="utf-8")
+        match = _VERSION_RE.search(text)
+        if match:
+            return match.group(1)
+    except (OSError, UnicodeDecodeError):
+        pass
+    try:
+        from src import __version__ as imported
+        return imported
+    except Exception:            # noqa: BLE001 - never let this raise
+        return "0.0.0"
+
+
 def parse_semver(value: Any) -> Optional[Tuple[int, int, int]]:
     """Parse ``X.Y.Z`` (extra parts and suffixes ignored) into a comparable
     3-tuple, or ``None`` when unparseable. A leading ``v`` is tolerated."""