fix(pm): keep streamed build-backend output without uv's DEBUG spew

The previous commit dropped --verbose and broke the contract that native
build output streams live (tests/pm/test_environment_build.py). uv only
forwards backend output at DEBUG level, so keep --verbose and scope it with
RUST_LOG=uv_build_frontend=debug: the backend's own lines stream, the
interpreter/cache/reflink internals stay silent (2 lines on a no-op sync
instead of ~200 on both uv 0.11 and the pinned 0.12.3).

pm/environment.py

@@ -262,9 +262,12 @@ class PythonEnvironment:
                 command.append("--offline")
             try:
                 if self.output is not None:
-                    # No --verbose: it is uv's DEBUG level and buries the progress
-                    # lines under interpreter/cache internals on every streamed run.
-                    # Build-backend output still arrives with the failure.
+                    # uv hides build-backend output until failure without verbose mode,
+                    # but --verbose alone is uv's DEBUG level: ~200 lines of interpreter
+                    # and cache internals on every streamed run. RUST_LOG scopes it to the
+                    # build frontend, so only the backend's own lines reach the user.
+                    command.append("--verbose")
+                    env.setdefault("RUST_LOG", "uv_build_frontend=debug")
                     return _run_streaming(command, cwd=cwd, env=env, timeout=timeout, output=self.output)
                 return subprocess.run(command, cwd=str(cwd), env=env, capture_output=True,
                                       text=True, encoding="utf-8", errors="replace", timeout=timeout)

tests/pm/test_index_bridging.py

@@ -57,15 +57,19 @@ def test_streamed_runs_do_not_request_uv_debug_output(tmp_path, monkeypatch):
     from pm import environment
 
     seen: list[list[str]] = []
+    kwargs_seen: list[dict] = []
 
     def record(command, **kwargs):
         seen.append(command)
+        kwargs_seen.append(kwargs)
         return subprocess.CompletedProcess(command, 0, "", "")
 
     monkeypatch.setattr(environment, "_run_streaming", record)
     PythonEnvironment(uv=tmp_path / "uv", python=tmp_path / "python", destination=tmp_path / "venv",
                       cache=tmp_path / "cache", env={}, output=io.StringIO())._run(["sync"], cwd=tmp_path, timeout=5)
-    assert seen and not {"--verbose", "-v"} & set(seen[0])
+    (command,), (kwargs,) = seen, kwargs_seen
+    # Verbose only where the build backend speaks; uv's own DEBUG stays silent.
+    assert "--verbose" in command and kwargs["env"]["RUST_LOG"] == "uv_build_frontend=debug"
 
 
 def test_uv_timeout_names_the_mirror_knobs(tmp_path, monkeypatch):