chore: untrack tsconfig.tsbuildinfo, and hold the CI baseline the way the local gate does

`frontend/tsconfig.tsbuildinfo` was swept into the previous commit by a `git add
-A`. It is a machine-local incremental artifact carrying absolute Windows paths,
no lane reads or regenerates it, and it was already stale in the commit that added
it — it does not list the contract test that same commit created. Untracked, and
`*.tsbuildinfo` ignored, because the day anything does start rewriting it a green
gate would leave a tracked file dirty: the exact hazard the basedpyright baseline
just got a guard for.

And CI ran bare basedpyright while the local lane gained that guard, so a
burn-down would have been green on the server and red on the developer's machine
over the same tree. Two gates, one tree, one answer.

.gitignore

@@ -37,3 +37,7 @@ backups/
 Thumbs.db
 .idea/
 .vscode/
+
+# TypeScript incremental build artifacts — machine-local, absolute-path-bearing, and no gate lane
+# reads or regenerates them, so a committed one is stale from the commit that adds it.
+*.tsbuildinfo