paper f2a: reuse-topology decomposition + mixture-sensitivity sweep

Full-trace analysis backing figure 2a on the real 2h cluster trace:

- f2a_reuse_topology_analyze.py: infinite-KV-cache (LRU) decomposition of
  prefix-cache reuse hits into intra-session vs cross-session, by most-recent
  prior holder of each content-addressed block.
- f2a_mixture_sweep.py: sensitivity of the intra/cross split to the
  single-turn session fraction (tests whether the 93%-intra sample vs 54.6%
  full-trace gap is session-mixture selection bias) -- keep all multi-turn
  sessions, downsample single-turn to each target fraction, reclassify.

Includes the result JSONs for both.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

paper/data/f2a_mixture_sweep_result.json

@@ -0,0 +1,84 @@
+{
+  "rows": [
+    {
+      "target": "full",
+      "single_turn_frac": 0.9026,
+      "n_sessions": 1307276,
+      "new": 20650883,
+      "intra": 65166144,
+      "cross": 54134925,
+      "reuse": 119301069,
+      "intra_frac_of_reuse": 0.5462,
+      "cross_frac_of_reuse": 0.4538,
+      "intra_le60s": 0.8865,
+      "cross_le60s": 0.8706
+    },
+    {
+      "target": 0.75,
+      "single_turn_frac": 0.75,
+      "n_sessions": 509144,
+      "new": 15446415,
+      "intra": 66081759,
+      "cross": 26932604,
+      "reuse": 93014363,
+      "intra_frac_of_reuse": 0.7104,
+      "cross_frac_of_reuse": 0.2896,
+      "intra_le60s": 0.8844,
+      "cross_le60s": 0.8568
+    },
+    {
+      "target": 0.5,
+      "single_turn_frac": 0.5,
+      "n_sessions": 254572,
+      "new": 12843712,
+      "intra": 66548474,
+      "cross": 18990485,
+      "reuse": 85538959,
+      "intra_frac_of_reuse": 0.778,
+      "cross_frac_of_reuse": 0.222,
+      "intra_le60s": 0.8832,
+      "cross_le60s": 0.8881
+    },
+    {
+      "target": 0.25,
+      "single_turn_frac": 0.25,
+      "n_sessions": 169715,
+      "new": 11553493,
+      "intra": 66732961,
+      "cross": 16726772,
+      "reuse": 83459733,
+      "intra_frac_of_reuse": 0.7996,
+      "cross_frac_of_reuse": 0.2004,
+      "intra_le60s": 0.8827,
+      "cross_le60s": 0.9087
+    },
+    {
+      "target": 0.1,
+      "single_turn_frac": 0.1,
+      "n_sessions": 141429,
+      "new": 11036894,
+      "intra": 66798704,
+      "cross": 16084035,
+      "reuse": 82882739,
+      "intra_frac_of_reuse": 0.8059,
+      "cross_frac_of_reuse": 0.1941,
+      "intra_le60s": 0.8826,
+      "cross_le60s": 0.9152
+    },
+    {
+      "target": 0.0,
+      "single_turn_frac": 0.0,
+      "n_sessions": 127286,
+      "new": 10724167,
+      "intra": 66834552,
+      "cross": 15799085,
+      "reuse": 82633637,
+      "intra_frac_of_reuse": 0.8088,
+      "cross_frac_of_reuse": 0.1912,
+      "intra_le60s": 0.8825,
+      "cross_le60s": 0.9184
+    }
+  ],
+  "n_single": 1179990,
+  "n_multi": 127286
+}