feat(store): refuse to install a plugin that needs a newer core

`ledmatrix_min_version` was decoration. The loader logged an advisory warning
and continued; the store never compared the core version at all, so a routine
"update" delivered a plugin that could not run. That is the gap phase B6 (the
sports-unification sunset) cannot be done over: deleting a plugin's bundled
fallback while nothing enforces the floor hands un-updated users a scoreboard
that raises ModuleNotFoundError at load and is reported only as one line in
the journal.

The gate lives in install_plugin, after the manifest is on disk and before
dependencies are installed. That is the earliest knowable point -- the
registry carries no compatibility field, so the floor is not visible until
the files are down -- and it is also the chokepoint: _reinstall_with_rollback
calls install_plugin, so a refused *update* restores the version the user
already had, for free.

Floor resolution and the comparison move to src/plugin_system/compatibility.py,
shared with the loader so the two cannot drift. Both read all four spellings
published manifests use, including the deprecated `ledmatrix_min`.

Refusal requires evidence. An undeclared floor, an unparseable version on
either side, or a core below TRUSTWORTHY_FLOOR (2.0.0) all allow the install.
That last one is deliberate and load-bearing: the v3.1.0 release reports
__version__ = "1.0.0" while nearly every published manifest floors at 2.0.0,
so a strict gate would lock those users out of the plugin store entirely --
much worse than the problem being solved. They stay unprotected until they
update the core, which is also what fixes their version string.

Verified: 782 core unit tests pass, including 25 new ones and the existing
loader-warning suite unchanged (the refactor is behavior-preserving). The
install tests drive the real install_plugin path with the download stubbed --
the allow and refuse cases differ only in the declared floor, so the refusal
is demonstrably the gate and not an earlier bail-out.

Follow-ups, deliberately not in this PR: surfacing the reason in the store UI
rather than only the log, and publishing the floor in plugins.json so the
store can refuse before downloading.

Phase B4 in docs/SPORTS_UNIFICATION.md.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Udr6MfaFLUPhX5Fgo67Jf5

src/plugin_system/compatibility.py

@@ -0,0 +1,102 @@
+"""One place that answers "can this plugin run on this core?".
+
+Two callers ask that question and they must not drift apart:
+
+- `PluginLoader._warn_if_incompatible` — at load time, **advisory**. A plugin
+  already on disk keeps loading regardless, because the guarded-import pattern
+  means most incompatibilities degrade rather than break.
+- `PluginStoreManager.install_plugin` — at install/update time, **blocking**.
+  This is the point where refusing costs the user nothing (they keep the
+  version they already had) and allowing can cost them a plugin that fails to
+  load with only a log line to explain it.
+
+## The trustworthiness problem
+
+The core's own `__version__` has not always been right. `v3.1.0` was tagged
+2026-05-31 while `src/__init__.py` still said `"1.0.0"`; the bump landed
+2026-07-12. Devices installed from that release report `1.0.0` — below the
+floor that essentially every published plugin declares.
+
+So a core reporting a version below `TRUSTWORTHY_FLOOR` is treated as
+**unknown, not old**: it neither warns nor blocks. Blocking on it would be far
+worse than the problem being solved — nearly every manifest in the ecosystem
+floors at `2.0.0`, so a strict gate would stop those users installing *any*
+plugin. They are unprotected until they update the core, which is also what
+fixes their version string. See `docs/SPORTS_UNIFICATION.md`, phase B4.
+"""
+
+from __future__ import annotations
+
+from typing import Any, Dict, Optional, Tuple
+
+# Below this, the core's self-reported version is not evidence of anything.
+# See the module docstring.
+TRUSTWORTHY_FLOOR: Tuple[int, int, int] = (2, 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."""
+    if not isinstance(value, str):
+        return None
+    parts = value.strip().lstrip('v').split('.')
+    try:
+        nums = [int(''.join(ch for ch in p if ch.isdigit()) or 0) for p in parts[:3]]
+    except ValueError:
+        return None
+    while len(nums) < 3:
+        nums.append(0)
+    return tuple(nums)  # type: ignore[return-value]
+
+
+def declared_min_version(manifest: Dict[str, Any]) -> Optional[str]:
+    """The core version this plugin says it needs, or ``None`` if it doesn't say.
+
+    Checked in order of specificity. `ledmatrix_min` is the deprecated spelling
+    of `ledmatrix_min_version` (`store_manager._validate_manifest_fields` flags
+    it); both are read because a large share of published manifests still carry
+    the old one.
+    """
+    declared = (
+        manifest.get('min_ledmatrix_version')
+        or (manifest.get('requires') or {}).get('min_ledmatrix_version')
+    )
+    if declared:
+        return declared
+
+    versions = manifest.get('versions') or []
+    if versions and isinstance(versions[0], dict):
+        return (versions[0].get('ledmatrix_min_version')
+                or versions[0].get('ledmatrix_min'))
+    return None
+
+
+def check(manifest: Dict[str, Any], core_version: str) -> Tuple[bool, Optional[str]]:
+    """Return ``(compatible, reason)``.
+
+    ``compatible`` is False **only** when the plugin declares a parseable floor,
+    the core reports a parseable and trustworthy version, and the floor is
+    genuinely above it. Every uncertain case resolves to compatible: an
+    undeclared floor, an unparseable version on either side, or a core whose
+    version is below `TRUSTWORTHY_FLOOR`. Refusing on a guess would break
+    working installs, which is the more expensive mistake here.
+
+    ``reason`` is user-facing text, present only when incompatible.
+    """
+    declared = declared_min_version(manifest)
+    needed = parse_semver(declared)
+    if needed is None:
+        return True, None
+
+    current = parse_semver(core_version)
+    if current is None or current < TRUSTWORTHY_FLOOR:
+        return True, None
+
+    if needed > current:
+        name = manifest.get('name') or manifest.get('id') or 'This plugin'
+        return False, (
+            f"{name} requires LEDMatrix {declared} or newer, but this system is "
+            f"running {core_version}. Update LEDMatrix first, then install it."
+        )
+
+    return True, None