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>

Makefile

@@ -3,7 +3,7 @@ GODOT := $(shell command -v godot4 2>/dev/null || command -v godot 2>/dev/null)
 .PHONY: help setup build client server game atlas stop test test-tooling lint lint-python setup-venv ci ci-client ci-server clean \
        decisions-sync decisions-active decisions-validate \
        validate-content check-fact-ids setup-hooks install-hooks \
-       audit deny atlas-verify economy-db regen-db check-systems-db \
+       audit deny atlas-verify economy-db regen-db check-systems-db check-client-version \
        pre-pr pre-pr-lint pre-pr-build pre-pr-test pre-pr-validate pre-pr-fixtures \
        pre-pr-server pre-pr-client pre-pr-content \
        fixtures-client fixtures-gauntlet golden-diff golden-update \
@@ -56,6 +56,7 @@ help:
 	@echo "  make economy-db        Import economics data into systems.db (TOML/JSON → SQLite)"
 	@echo "  make regen-db          Regenerate systems.db from all sources + stamp meta table (#855)"
 	@echo "  make check-systems-db  Verify systems.db meta stamp matches current generator sources"
+	@echo "  make check-client-version  Verify client/project.godot version mirrors project.yaml"
 	@echo "  make install-hooks     Install pre-push + pre-commit git hooks (once per clone)"
 	@echo "  make fixtures-client    Generate GDScript->Rust cross-encoder fixtures (#475)"
 	@echo "  make golden-diff        Show diff if golden file output has changed"
@@ -406,6 +407,9 @@ regen-db:  ## Regenerate systems.db from all sources and stamp meta table (#855,
 check-systems-db:  ## Verify systems.db meta stamp matches current generator sources (#857)
 	@python3 tooling/check-systems-db-stamp --verbose
 
+check-client-version:  ## Verify client/project.godot's baked version matches project.yaml (T-1241)
+	@python3 tooling/check-client-version
+
 econ-sim:  ## Build the economics simulation binary (Layer 1+2: Leontief + tâtonnement trade)
 	@cargo build --manifest-path tooling/econ-sim/Cargo.toml --release
 	@echo "Built: tooling/econ-sim/target/release/econ-sim"