Stabilize first-run cudagraph timing and colorize terminal output

- _cudagraph_plugin: warm up autotune/JIT explicitly before graph capture
  (internal 5-iter warmup is too short), tag fallback markers with the
  failing phase; first-run latency no longer jitters run-to-run.
- _device_guard_plugin (new): set GEMS_VENDOR via torch probe before
  importing flag_gems, avoiding its timeout-less nvidia-smi subprocess
  probe that can hang import in fork-broken environments.
- _pretty_report_plugin (new) + _term_style (new): fold inputs identical
  across all result rows into a legend line, color status/plugin
  tags/markers on the live terminal; run.log is ANSI-stripped and keeps
  upstream SUCCESS/column wording for grep compatibility.
- run_pytest.sh: make USE_FLAGTUNE overridable, group all knobs into a
  config section with Chinese comments, add start/end banners.
- README: document the warmup semantics, new plugins/env vars, and the
  A/B rule that both sides must use the same USE_FLAGTUNE.

_device_guard_plugin.py

@@ -0,0 +1,45 @@
+"""Guard against FlagGems' device-detection hang (benchmark-side fix).
+
+FlagGems' `runtime.backend.device_finder` falls back to
+`subprocess.run(nvidia-smi)` **without a timeout**. Under this conda env's
+forked/inconsistent subprocess (`_posixsubprocess` symbol mismatch), that
+child can hang indefinitely, leaving `import flag_gems` stuck in `wait4`
+(seen as run_pytest "卡住 with no result", GPU 0%).
+
+This plugin runs at import time — before any test module imports flag_gems —
+detects the vendor via torch (no subprocess), and sets GEMS_VENDOR so
+device_finder takes its env fast-path (`_get_vendor_from_env`) and never
+reaches the hang-prone subprocess probe. No change to the FlagGems repo.
+
+Side effect: get_device_properties initializes the CUDA context very early in
+the pytest process. Fine for the current single-process runs; revisit if
+fork-based parallelism (e.g. pytest-xdist) is ever introduced.
+"""
+import os
+import sys
+
+_VENDOR_ENV_KEYS = ("GEMS_VENDOR", "FLAGGEMS_VENDOR", "GEMS_BACKEND", "FLAGGEMS_BACKEND")
+
+
+def _guard_device_vendor():
+    # Respect an explicit choice if the user already set one.
+    if any(k in os.environ for k in _VENDOR_ENV_KEYS):
+        return
+    try:
+        import torch
+
+        if torch.cuda.is_available():
+            name = torch.cuda.get_device_properties(0).name.upper()
+            if "NVIDIA" in name:
+                os.environ["GEMS_VENDOR"] = "nvidia"
+                from _term_style import tag
+                print(f"{tag('[device-guard-plugin]')} set GEMS_VENDOR=nvidia "
+                      "(skip flag_gems nvidia-smi subprocess probe, avoids import hang)",
+                      file=sys.stderr, flush=True)
+    except Exception as e:  # pragma: no cover
+        print(f"[device-guard-plugin] torch vendor probe failed ({e}); "
+              "leaving detection to flag_gems",
+              file=sys.stderr, flush=True)
+
+
+_guard_device_vendor()