compute_roofline: argparse --trace, fix stale default path (D4)

The hardcoded traces/sampled_1000req_seed42.jsonl no longer exists; switch
the default to the current sampled trace file w600_r0.0015_st30.jsonl and
let users override via --trace. Skip Part 4 cleanly when the file is
missing instead of relying on os.path.exists.

scripts/compute_roofline.py

@@ -12,6 +12,7 @@ GPU: NVIDIA H20
   - Roofline ridge point: 148/4.0 = 37 FLOP/byte
 """
 
+import argparse
 import json
 import math
 
@@ -161,16 +162,25 @@ print("  PART 4: Agentic Workload Real Distribution")
 print("-" * 80)
 
 # Use actual trace data
-import os
-trace_path = "traces/sampled_1000req_seed42.jsonl"
-if os.path.exists(trace_path):
+_parser = argparse.ArgumentParser(description=__doc__)
+_parser.add_argument("--trace", type=str,
+                     default="traces/w600_r0.0015_st30.jsonl",
+                     help="Sampled trace JSONL for empirical workload roofline (Part 4)")
+_args, _ = _parser.parse_known_args()
+trace_path = _args.trace
+try:
+    _trace_fh = open(trace_path)
+except FileNotFoundError:
+    print(f"  (skipped: trace file not found: {trace_path})")
+    _trace_fh = None
+if _trace_fh is not None:
     BLOCK_SIZE = 512
     seen = set()
     compute_bound = 0
     memory_bound = 0
     total = 0
 
-    for line in open(trace_path):
+    for line in _trace_fh:
         d = json.loads(line)
         seq_len = d["input_length"]
         if seq_len < 1: continue
@@ -201,6 +211,8 @@ if os.path.exists(trace_path):
         else:
             memory_bound += 1
 
+    _trace_fh.close()
+    if total > 0:
         print(f"  With actual trace prefix cache pattern:")
         print(f"    Compute-bound prefills: {compute_bound} ({compute_bound*100//total}%)")
         print(f"    Memory-bound prefills:  {memory_bound} ({memory_bound*100//total}%)")