fix(client): bake the version into the build, so an export can invalidate its cache (T-1241)

current_schema_version() line-scanned res://../project.yaml at runtime. That
resolves to the repo root in a dev run and to nothing in an exported build, so a
shipped game got the "?.?.?" fallback every time. Since that tag is the Atlas
disk cache's ONLY invalidation signal, every exported build stamped and compared
the same sentinel: a canvas cached by one build would be served by every later
build, forever. T-1239 is what that failure looks like once it happens.

loading_screen.gd carried a byte-for-byte copy of the same function, so the
version shown to the player was "?.?.?" in exactly the builds where a version
string is worth showing. Both call sites now share client/scripts/build_version.gd,
which reads application/config/version out of ProjectSettings — a value Godot
bakes into the PCK, identical in the editor and in an export by construction
rather than by luck. No file IO, no fallback branch.

project.yaml stays the source of truth (CLAUDE.md); client/project.godot mirrors
it. A mirror nobody checks would be worse than the bug it replaces -- the old
code failed loudly everywhere, a stale mirror fails silently -- so
tooling/check-client-version compares the two and the pre-push hook runs it
unconditionally. Not gated on "were those files in this push": drift persists on
main once introduced, and gating would let an existing drift ride along.

The test this replaces asserted that current_schema_version() did not return its
fallback, and passed -- in the one environment where the code under test worked.
Three tests now pin the property that actually matters: a real version, sourced
from the baked setting, matching project.yaml.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.config/hooks/pre-push

@@ -278,6 +278,26 @@ else
     echo "pre-push: systems.db not in push — skipping stamp check"
 fi
 
+# --- Client version mirror (T-1241) ---
+# project.yaml is the version source of truth, but the client cannot read it at
+# runtime (an exported build has no repo root), so it is mirrored into
+# client/project.godot's `application/config/version`. The Atlas disk cache keys
+# its ONLY invalidation signal on that value, so a stale mirror makes a build
+# serve canvases generated by code it no longer runs — the T-1239 failure.
+#
+# Deliberately UNCONDITIONAL, unlike the systems.db check above. Drift is
+# introduced by touching one file without the other, but it PERSISTS on main
+# until someone notices, so gating on "was either file in this push" would let
+# an existing drift ride along indefinitely. The check is two file reads.
+if [ -f "$REPO_ROOT/tooling/check-client-version" ]; then
+    echo "pre-push: checking client version mirror..."
+    if python3 "$REPO_ROOT/tooling/check-client-version"; then
+        :
+    else
+        fail_check "client version mirror (drifted from project.yaml)"
+    fi
+fi
+
 # --- Clerk review (D-221) ---
 # DISABLED 2026-05-23 (#965) pending rework. Two problems made it net-negative:
 #   1. Non-exhaustive — a single run reports ~the first contradiction it finds