Record the stale-build-tree coverage defect

Defects #13: gcovr searches for gcov data under --root, which
CMakeLists.txt:208 and :223 set to the source directory, so any
instrumented build tree left there is merged into the report. The
--object-directory argument beside each does not narrow the search; it
only maps gcda files back to the compiler's working directory.

A leftover build-coverage/ therefore fails coverage_reset before any
test runs, with gcovr unable to reconcile a function reported on two
different lines. The build*/ entry in .gitignore keeps the offending
tree out of git status, so the state is easy to get into and hard to
spot.

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

TODO.md

@@ -586,6 +586,35 @@ Each was found by a test written to assert correct behavior.
     dedicated `controllerdb` target, or an `OUTPUT` that matches where the
     script actually writes — so an ordinary build neither needs the network nor
     dirties the tree.
+13. **A stale build tree in the source directory breaks the coverage run.**
+    `CMakeLists.txt:208` and `CMakeLists.txt:223` pass
+    `--root "${CMAKE_CURRENT_SOURCE_DIR}"` to gcovr, and gcovr searches for
+    `.gcda`/`.gcno` files under the root. The `--object-directory` argument on
+    the line below each does *not* narrow that search: per `gcovr --help` it
+    only identifies "the path between gcda files and the directory where the
+    compiler was originally run". So every instrumented build tree left inside
+    the source directory is folded into the report alongside the one actually
+    being measured.
+
+    With a `build-coverage/` from an earlier session still present, a freshly
+    configured `-DAKGL_COVERAGE=ON` tree fails in `coverage_reset`, before any
+    test runs:
+
+        AssertionError: Got function akgl_game_lowfps on multiple lines: 45, 46.
+
+    45 and 46 are that function's line numbers before and after an unrelated
+    `#include` was added to `src/game.c`: gcovr found 18 stale `.gcno` files
+    describing the old layout, merged them with the current ones, and could not
+    reconcile the two. Moving `build-coverage/` aside makes the same tree pass
+    18/18. Verified with gcovr 7.0.
+
+    The `build*/` entry in `.gitignore` hides these trees from `git status`,
+    which makes the state easier to get into and no easier to notice.
+
+    Fix: pass the build tree to gcovr as an explicit search path instead of
+    letting it default to `--root`, so only the tree under measurement is
+    considered. Touches the two `add_test` blocks at `CMakeLists.txt:204-211`
+    and `CMakeLists.txt:220-229`; nothing outside the `AKGL_COVERAGE` branch.
 
 ## Build notes