feat(analyzer): surface TTFT/TPOT as features and CDF figures

The gateway records three epoch-ms stamps per request (admitted, forwarded to
backend, backend first token). They reached *-raw.jsonl but stopped there:
features.csv had only total latency, and none of the figures covered
time-to-first-token.

Derive ttft_ms, ttft_e2e_ms and tpot_ms in one helper shared by the streaming
path (preparation.py) and the in-memory path (features.py) so the two cannot
drift, and add figures 14 (TTFT, backend vs end-to-end) and 15 (TPOT). The two
TTFT series are plotted together because their offset is the gateway admission
delay, which is then readable straight off the chart.

Rows missing a backend stamp and single-token responses are recorded as 0 and
filtered at plot time; left in, they would stack at the origin and pull every
percentile down. Figure 15 is a single panel: TPOT is near-deterministic
(p50 21.9, p90 23.0), so a p90 zoom renders a flat line and a vertical edge.

Verified by recomputing ttft_ms straight from the original SLS rows for all
2190 requests of a sample: 0 mismatches.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

trace_analyzer/figures.py

@@ -44,6 +44,8 @@ FIGURE_STEMS = [
     "11_alive_kvcache_blocks_timeline",
     "12_bucket_kvcache_reuse_ratio",
     "13_session_cross_bucket_kvcache_miss",
+    "14_ttft_cdf",
+    "15_tpot_cdf",
 ]
 
 
@@ -112,6 +114,9 @@ def _load_request_metrics(path: Path) -> list[dict]:
                     "output_tokens": safe_int(row.get("output_tokens")),
                     "request_ready_time_ms": safe_int(row.get("request_ready_time_ms")),
                     "request_end_time_ms": safe_int(row.get("request_end_time_ms")),
+                    "ttft_ms": safe_int(row.get("ttft_ms")),
+                    "ttft_e2e_ms": safe_int(row.get("ttft_e2e_ms")),
+                    "tpot_ms": safe_float(row.get("tpot_ms")),
                     "input_length_bucket": row.get("input_length_bucket", ""),
                     "theoretical_prompt_unit_length": safe_int(row.get("theoretical_prompt_unit_length")),
                     "theoretical_prefix_hit_blocks": safe_int(row.get("theoretical_prefix_hit_blocks")),
@@ -623,6 +628,42 @@ def _plot_session_cross_bucket_miss(fig_dir: Path, rows: list[dict]) -> None:
     _save(fig, fig_dir, "13_session_cross_bucket_kvcache_miss")
 
 
+def _plot_ttft_cdf(fig_dir: Path, request_rows: list[dict]) -> None:
+    # Requests missing either backend timestamp carry 0. Plotting those would pile
+    # a spike at the origin and drag every percentile down, so drop them.
+    backend_values = [row["ttft_ms"] for row in request_rows if row["ttft_ms"] > 0]
+    e2e_values = [row["ttft_e2e_ms"] for row in request_rows if row["ttft_e2e_ms"] > 0]
+    _plot_two_series_cdf_with_zoom(
+        fig_dir,
+        stem="14_ttft_cdf",
+        title="Time To First Token CDF",
+        xlabel="Milliseconds",
+        first_label="TTFT backend",
+        first_values=backend_values,
+        first_color=PALETTE["teal"],
+        second_label="TTFT end-to-end",
+        second_values=e2e_values,
+        second_color=PALETTE["red"],
+        zoom_quantile=0.90,
+        stats_labels=("mean", "p50", "p90", "p95", "p99"),
+    )
+
+
+def _plot_tpot_cdf(fig_dir: Path, request_rows: list[dict]) -> None:
+    # A single-token response has no decode interval to average over and is
+    # reported as 0; it is not a zero-cost token.
+    values = [row["tpot_ms"] for row in request_rows if row["tpot_ms"] > 0]
+    _plot_single_cdf(
+        fig_dir,
+        stem="15_tpot_cdf",
+        title="Time Per Output Token CDF",
+        xlabel="Milliseconds per output token",
+        label="TPOT",
+        values=values,
+        color=PALETTE["green"],
+    )
+
+
 def _write_manifest(fig_dir: Path, manifest: dict) -> None:
     (fig_dir / "manifest.json").write_text(json.dumps(manifest, ensure_ascii=False, indent=2), encoding="utf-8")
 
@@ -781,6 +822,14 @@ def render_figures(
         progress.update(1)
         progress.set_postfix(current="13_session_cross_bucket_kvcache_miss")
     _plot_session_cross_bucket_miss(fig_root, session_bucket_rows)
+    if show_progress:
+        progress.update(1)
+        progress.set_postfix(current="14_ttft_cdf")
+    _plot_ttft_cdf(fig_root, request_rows)
+    if show_progress:
+        progress.update(1)
+        progress.set_postfix(current="15_tpot_cdf")
+    _plot_tpot_cdf(fig_root, request_rows)
     _plot_session_gap_cdf(fig_root, session_rows_by_id)
 
     if show_progress: