fix(version): make the core version have exactly one answer

Plugin compatibility floors compare against src.__version__, so that string
has to be trustworthy. It has not been. v3.1.0 was tagged 2026-05-31 while
src/__init__.py still said "1.0.0"; the bump did not land until 2026-07-12.
Every device installed from that release reports 1.0.0, which is below the
(2, 0, 0) floor in PluginLoader._warn_if_incompatible -- so those users are
silently exempt from every plugin compatibility warning.

web_interface carried a third answer, a hardcoded "3.0.0" that nothing read
and that had drifted two majors from the core. It now re-exports the
canonical value, so it cannot disagree again.

Adds:

- test/test_version_consistency.py (enrolled in the core unit CI job):
  src.__version__ is parseable semver, matches the newest CHANGELOG heading,
  the CHANGELOG's headings are unique and descending, and web_interface
  tracks the core. src.plugin_system.__version__ is deliberately excluded --
  it versions the plugin API and moves independently.

- scripts/check_release_version.py + a release-version-check workflow that
  asserts the tag, the CHANGELOG and src.__version__ agree. Runs on pushed
  v* tags and published releases, and via workflow_dispatch so a tag can be
  checked *before* it is created:

      python scripts/check_release_version.py v3.2.0

Verified: 757 core unit tests pass including the four new ones; the script
exits 0 for v3.2.0 and non-zero for both a mismatched tag (v3.1.0) and a
non-semver one (v2.5); web_interface and web_interface.app still import.

Prerequisite for cutting v3.2.0 -- 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

scripts/check_release_version.py

@@ -0,0 +1,99 @@
+#!/usr/bin/env python3
+"""Assert that a release tag, the CHANGELOG, and `src.__version__` all agree.
+
+Run it *before* creating a tag to check yourself:
+
+    python scripts/check_release_version.py v3.2.0
+
+CI runs it on every pushed `v*` tag and published release
+(`.github/workflows/release-version-check.yml`), so a mismatch shows up as a
+red check on the release rather than as a silent wrong answer on user devices.
+
+Why this exists: `v3.1.0` was tagged 2026-05-31 while `src/__init__.py` still
+said `"1.0.0"`; the bump to `"3.1.0"` did not land until 2026-07-12. Devices
+installed from that release report `1.0.0`, which is below the `(2, 0, 0)` floor
+in `PluginLoader._warn_if_incompatible`, so they are silently exempt from every
+plugin compatibility warning. Plugin `ledmatrix_min_version` floors are only as
+trustworthy as this agreement. See `docs/SPORTS_UNIFICATION.md`, phase B4.
+"""
+
+from __future__ import annotations
+
+import argparse
+import re
+import sys
+from pathlib import Path
+
+REPO_ROOT = Path(__file__).resolve().parents[1]
+sys.path.insert(0, str(REPO_ROOT))
+
+SEMVER = re.compile(r"^\d+\.\d+\.\d+$")
+HEADING = re.compile(r"^##\s+(?P<version>\d+\.\d+\.\d+)\s*$", re.MULTILINE)
+
+
+def normalize(tag: str) -> str:
+    """`v3.2.0` and `3.2.0` are the same release; tags here carry the `v`."""
+    return tag[1:] if tag.startswith("v") else tag
+
+
+def newest_changelog_version(changelog: Path) -> str | None:
+    headings = HEADING.findall(changelog.read_text(encoding="utf-8"))
+    return headings[0] if headings else None
+
+
+def main() -> int:
+    parser = argparse.ArgumentParser(description=__doc__)
+    parser.add_argument(
+        "tag",
+        help="Release tag to check, with or without the leading 'v' (e.g. v3.2.0)",
+    )
+    args = parser.parse_args()
+
+    from src import __version__ as core_version
+
+    tag_version = normalize(args.tag)
+    changelog_version = newest_changelog_version(REPO_ROOT / "CHANGELOG.md")
+
+    problems: list[str] = []
+
+    if not SEMVER.match(tag_version):
+        problems.append(
+            f"tag {args.tag!r} is not vX.Y.Z. Older tags (v2.5) predate this "
+            "check; new releases must be full semver so floors can parse them."
+        )
+
+    if not SEMVER.match(core_version):
+        problems.append(f"src.__version__ is {core_version!r}, which is not X.Y.Z")
+
+    if tag_version != core_version:
+        problems.append(
+            f"tag says {tag_version} but src.__version__ says {core_version}. "
+            "Bump src/__init__.py to match the tag before releasing — devices "
+            "report __version__, not the tag, and plugin floors compare "
+            "against it."
+        )
+
+    if changelog_version is None:
+        problems.append("CHANGELOG.md has no '## X.Y.Z' version heading")
+    elif changelog_version != core_version:
+        problems.append(
+            f"CHANGELOG.md's newest heading is {changelog_version} but "
+            f"src.__version__ is {core_version}. Plugin authors read the "
+            "CHANGELOG to pick a ledmatrix_min_version floor."
+        )
+
+    if problems:
+        print(f"Release version check FAILED for tag {args.tag}:", file=sys.stderr)
+        for problem in problems:
+            print(f"  - {problem}", file=sys.stderr)
+        return 1
+
+    print(
+        f"OK: tag {args.tag}, src.__version__ {core_version}, and the CHANGELOG "
+        "all agree."
+    )
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())