test(stable-mir-ui): add external UI test harness with skip management (#968)

## Summary

- Add parametrized pytest harness running `kmir prove-rs` against every
entry in `stable-mir-json`'s `passing.tsv`
- Ship `skip.txt` (2859 entries) for known-failing cases, with
`--update-skip` mode to shrink it by re-proving skipped cases
- 300s per-test timeout via `pytest-timeout`; proof show output saved to
`tmp_path` on failure

## Usage

```bash
# Normal run (skipped cases are skipped)
RUST_DIR_ROOT=/path/to/rust make test-stable-mir-ui

# Shrink skip.txt by re-proving skipped cases
RUST_DIR_ROOT=/path/to/rust make test-stable-mir-ui TEST_ARGS="--update-skip"
```

## Test plan

- [x] `make check` passes (B011 lint fixed)
- [x] Single squashed commit, rebased on `origin/master`

---------

Co-authored-by: Daniel Cumming <124537596+dkcumming@users.noreply.github.com>
Co-authored-by: dkcumming <daniel.cumming@runtimeverification.com>

Author: Stevengre

Makefile

@@ -56,6 +56,11 @@ test-integration: stable-mir-json build
 	$(UV_RUN) pytest $(TOP_DIR)/kmir/src/tests/integration --maxfail=1 --verbose \
 			--durations=0 --numprocesses=$(PARALLEL) --dist=worksteal $(TEST_ARGS)
 
+.PHONY: test-stable-mir-ui
+test-stable-mir-ui: stable-mir-json build
+	@test -n "$(RUST_DIR_ROOT)" || (echo "RUST_DIR_ROOT is required. Example: RUST_DIR_ROOT=/path/to/rust make test-stable-mir-ui"; exit 2)
+	$(UV_RUN) pytest $(TOP_DIR)/kmir/src/tests/external/test_stable_mir_ui_pass.py --maxfail=1 --verbose $(TEST_ARGS)
+
 # Checks and formatting
 
 format: autoflake isort black nix-fmt

kmir/pyproject.toml

@@ -42,6 +42,7 @@ dev = [
     "pytest",
     "pytest-cov",
     "pytest-mock",
+    "pytest-timeout",
     "pytest-xdist",
     "pyupgrade",
 ]

kmir/src/tests/external/conftest.py

@@ -0,0 +1,22 @@
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+import pytest
+
+if TYPE_CHECKING:
+    from pytest import FixtureRequest, Parser
+
+
+def pytest_addoption(parser: Parser) -> None:
+    parser.addoption(
+        '--update-skip',
+        action='store_true',
+        default=False,
+        help='Shrink stable-mir-ui skip entries by rerunning only current skip.txt cases.',
+    )
+
+
+@pytest.fixture(scope='session')
+def update_skip_mode(request: FixtureRequest) -> bool:
+    return request.config.getoption('--update-skip')