compiler: remove the implicit last-build state (B2)

generate_config/compile_kernel/load_module also recorded the last build
directory and PTX in ContextVars so that bare compile_kernel() and
load_module() calls worked: a second, implicit interface beside the
explicit build_dir/ptx_path one used by every owner. compile_kernel now
requires build_dir= and load_module requires the PTX path.

Build contract tests use explicit paths; they still check per-build
header/output isolation, interleaved generation, path escapes, refusal of a
removed snapshot, and that a failed compile keeps the previous PTX and
leaves no partial output. No kernel, numerical or launch change.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/unit/test_build_contracts.py

@@ -126,8 +126,7 @@ class BuildContracts(unittest.TestCase):
             def work(nx, count):
                 build = compiler.generate_config(LBMConfig(nx=nx), count, build_root=root)
                 barrier.wait()
-                # Compatibility form is context-local even without explicit path.
-                output = compiler.compile_kernel()
+                output = compiler.compile_kernel(build_dir=build)
                 data = json.loads(Path(output).read_text())
                 self.assertEqual(data, {"nx": nx, "objects": count})
                 metadata = json.loads(Path(output + ".json").read_text())
@@ -156,7 +155,7 @@ class BuildContracts(unittest.TestCase):
             compiler.cleanup_build_dir(a)
             compiler.cleanup_build_dir(b)
             with self.assertRaises(RuntimeError):
-                compiler.compile_kernel()
+                compiler.compile_kernel(build_dir=a)  # removed snapshot is never compiled
 
     def test_failed_compile_cannot_implicitly_load_previous_artifact(self):
         with tempfile.TemporaryDirectory() as root:
@@ -169,8 +168,7 @@ class BuildContracts(unittest.TestCase):
                 with self.assertRaises(subprocess.CalledProcessError):
                     compiler.compile_kernel(build_dir=build)
             self.assertEqual(Path(previous).read_bytes(), old_bytes)
-            with self.assertRaisesRegex(RuntimeError, "No compiled PTX"):
-                compiler.load_module()
+            self.assertEqual(sorted(Path(build).glob("kernel_v2-*.ptx")), [])  # no partial output
             compiler.cleanup_build_dir(build)