simulation/lab: open exp-010 with the splitting-pressure baseline

In this commit, we record the pre-evolution baseline on the corridors
corpus, and it already contains a finding: this is the first
environment where lnd's production stack beats members of the evolved
lineage. Its divide-and-conquer MPP completes 96% of the held-out
payments -- second-best objective despite burning 23 attempts per
payment -- while the naive seed manages 75% and even gen2 trails it.
mx_c3's evidence-derived shard ladder still leads at 0.876, but at
ten attempts per payment there is clear headroom for a router that
plans its route set up front instead of discovering shard sizes by
failure. The code_split1 run now chases exactly that, with the
reflection prompt naming joint route-set planning as the unexplored
axis.

simulation/lab/experiments/exp-010-splitting-pressure.md

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+# EXP-010 — Splitting pressure: does joint route-set planning emerge?
+
+**Date:** 2026-07-25 (started)
+**Status:** in flight — baseline done, evolution run `code_split1` live
+
+## Question
+Every winner so far splits reactively: try an amount, and when it
+fails, carve the next shard from a ladder of halves and
+evidence-derived sizes. Nobody has evolved joint route-set planning —
+choosing a set of routes AND their shard amounts together,
+min-cost-flow style. Does it emerge when the environment makes
+deliberate unequal splits the difference between success and failure?
+
+## Environment
+`corpus-split` (seed 4041), built on the new corridors topology
+(commit 11f4ccc65): K = 8–16 parallel corridors of deliberately
+unequal capacity tiers (one fat corridor, then rungs each at most half
+its size) between one source and one target, with the tier enforced
+structurally by the target-inbound channel capacity — the fattest tier
+is a hard ceiling on any single shard, the tier sum a hard ceiling on
+the payment. Bimodal liquidity, no drift (one variable at a time).
+Each file: two cheap probes that seed corridor knowledge, then one
+ambitious payment above the fattest tier. A forced max_parts=1 control
+fails 40/40 files: splitting is mandatory by construction, and the
+uneven ladder makes the right split unequal — halving an above-tier
+payment yields shards only the fat corridor can carry.
+
+## Baseline (before evolution)
+
+| router | split-val obj | split-test obj | test succ | test att |
+|---|---|---|---|---|
+| lnd stack | 0.782 | 0.837 | 0.958 | 23.4 |
+| seed | 0.594 | 0.644 | 0.750 | 20.1 |
+| hb1 | 0.814 | 0.814 | 0.917 | 12.1 |
+| **mx_c3** | **0.835** | **0.876** | 0.958 | 10.2 |
+| gen2 | 0.801 | 0.770 | 0.875 | 10.7 |
+| drift1 | 0.826 | 0.829 | 0.917 | 9.3 |
+
+Findings before evolution starts:
+- **This corpus reverses the usual ordering for lnd.** Its production
+  divide-and-conquer MPP is genuinely good at completing these
+  payments (0.958 success on test, second-best objective) — it just
+  pays 23.4 attempts/payment for it. On corpus-building verification
+  it beat the naive seed outright (0.79 vs 0.67 mean objective), the
+  first environment where lnd tops any evolved-lineage member.
+- **mx_c3's halving-plus leads**, consistent with its
+  evidence-derived shard ladder, but at ~10 attempts/payment there is
+  clear headroom: an efficient joint planner should complete these
+  payments in roughly half the attempts (the probes reveal corridor
+  tiers; sizing shards to tiers up front should rarely miss).
+- The seed's 0.594/0.644 shows the gradient the run gets to climb.
+
+## Evolution run
+`code_split1`: pure gepa, codex/gpt-5.6-sol reflection, small seed +
+insights prompt, 400 evals, corpus-split. The prompt names joint
+route-set planning as unexplored design space (commit 12276e6cf) and
+carries the exp-008 lesson so budget is not wasted rediscovering
+decay. Success criterion: beat mx_c3 on held-out split-test, then
+check the winner's structure — does it plan a route SET up front
+(min-cost-flow shape) or refine the reactive ladder further? Either
+answer is informative; a win by ladder refinement would suggest
+sequential adaptivity beats up-front planning even under maximal
+splitting pressure.
+
+## Verdict
+(pending run completion)