feat(gen): streaming + crash-resilient resume for full-scale data gen

The original gen_synthetic.py held all N samples in a [None]*n list and
only wrote at the end — three full-scale runs were killed mid-way (env
reaps long sessions) and lost everything (0 bytes on disk each time).

Fixes (proven on the 500k+100k run that just completed):
- run() now streams: each completed future is written to disk the instant
  it's produced, via an O(workers) reorder buffer that keeps output in
  index order. Peak RAM drops from O(n) (~2.7GB for 50k) to O(workers).
- --resume flag: counts existing lines and appends only the missing
  suffix (same seed => deterministic prefix, so resume is content-safe).
- scripts/gen_full_supervisor.sh: loops 'run --resume -> check count'
  until target reached. Each surviving iteration adds >=1 line, so the
  run converges even across multiple kills (took 2 kills + 2 resumes to
  finish the 500k align). On this host: 2 partial runs (50k + 437k) then
  a final resume filled the last 62k.
- Verified: order-deterministic (same-seed runs byte-identical), resume
  appends without rewriting the prefix, target-already-met => no-op.

Full-scale data now on disk: align 500k (27G) + sft 100k (5.4G) +
eval_synth 2k held-out (untouched).

scripts/gen_synthetic.py

@@ -22,10 +22,11 @@ import json
 import os
 from concurrent.futures import ProcessPoolExecutor, as_completed
 from pathlib import Path
+from typing import Any
 
 from tqdm import tqdm
 
-from tsmm.data.gen_pipeline import gen_one_sample, write_jsonl
+from tsmm.data.gen_pipeline import _to_jsonable, gen_one_sample, write_jsonl
 from tsmm.data.instruct import evolve_instruction
 
 
@@ -55,19 +56,53 @@ def run(
     missing_rate: float = 0.05,
     event_prob: float = 0.5,
     do_evolve: bool = False,
+    resume: bool = False,
 ) -> int:
+    """Stream samples to ``out`` as they are produced.
+
+    Writes each sample to disk the instant its future completes, while keeping
+    the output file in index order via a tiny reorder buffer (size <= workers).
+    Peak memory is thus O(workers) instead of O(n): 500k samples never all sit
+    in RAM, and a crash loses only the unwritten tail rather than everything.
+
+    With ``resume=True`` (and ``--n`` as the *target* total), count existing
+    lines in ``out`` and only generate the missing suffix, appending. Repeated
+    crashes converge to the full target: each resume picks up where it stopped.
+    """
     Path(out).parent.mkdir(parents=True, exist_ok=True)
+    start_idx = 0
+    mode = "w"
+    if resume and os.path.exists(out):
+        with open(out, "r", encoding="utf-8") as f:
+            start_idx = sum(1 for _ in f)
+        if start_idx >= n:
+            print(f"[gen] {out} already has {start_idx} lines >= target {n}; nothing to do")
+            return start_idx
+        mode = "a"
+        print(f"[gen] resume: {out} has {start_idx}/{n}, appending {n - start_idx}")
     work = [
         (T, C, seed + i, anomaly_types, missing_rate, event_prob, do_evolve)
-        for i in range(n)
+        for i in range(start_idx, n)
     ]
-    samples = [None] * n
-    with ProcessPoolExecutor(max_workers=workers) as ex:
-        futures = {ex.submit(_one, w): i for i, w in enumerate(work)}
-        for fut in tqdm(as_completed(futures), total=n, desc=os.path.basename(out)):
+    written = start_idx
+    pending: dict[int, Any] = {}   # index -> sample, awaiting its turn
+    next_i = start_idx              # next index that must be flushed
+    with open(out, mode, encoding="utf-8") as f, \
+         ProcessPoolExecutor(max_workers=workers) as ex:
+        futures = {ex.submit(_one, w): i for i, w in zip(range(start_idx, n), work)}
+        for fut in tqdm(as_completed(futures), total=len(work),
+                        desc=os.path.basename(out), initial=start_idx):
             i = futures[fut]
-            samples[i] = fut.result()
-    return write_jsonl(samples, out)
+            pending[i] = fut.result()
+            # flush everything that is now contiguously ready, in order
+            while next_i in pending:
+                f.write(json.dumps(_to_jsonable(pending.pop(next_i)),
+                                   ensure_ascii=False))
+                f.write("\n")
+                next_i += 1
+                written += 1
+            f.flush()
+    return written
 
 
 def main(argv: list[str] | None = None) -> int:
@@ -81,6 +116,8 @@ def main(argv: list[str] | None = None) -> int:
     p.add_argument("--missing-rate", type=float, default=0.05)
     p.add_argument("--event-prob", type=float, default=0.5)
     p.add_argument("--evolve", action="store_true", help="apply Evol-Instruct rephrasing (for sft set)")
+    p.add_argument("--resume", action="store_true",
+                   help="count existing lines in --out and append only the missing suffix (crash-safe)")
     args = p.parse_args(argv)
 
     n = run(
@@ -93,6 +130,7 @@ def main(argv: list[str] | None = None) -> int:
         missing_rate=args.missing_rate,
         event_prob=args.event_prob,
         do_evolve=args.evolve,
+        resume=args.resume,
     )
     print(f"wrote {n} samples -> {args.out}")
     return 0