society: make the sexual-transmission model rigorous (E9 epistasis, E10 directed sex)

Deepen the sexual-reproduction frame before entering the full society, on
the two facets GG chose: landscape robustness and directed recombination.
Adds a Kauffman NK landscape (genotype.nk_fitness, tunable ruggedness),
finite n-parent crossover (genotype.crossover, per-gap recombination rate),
and hill-climb (parents = local optima = trained models).

E9 (recomb_landscape) -- the "why sex?" test: E8's dramatic super-parent
result used an ADDITIVE landscape. On rugged/epistatic landscapes, blindly
recombining local optima causes OUTBREEDING DEPRESSION -- offspring fall
below the parents, worse with both ruggedness and recombination rate (K=8,
free recomb: ~ -0.23), and the optimal recombination rate shrinks as
ruggedness grows. Design rule: merge freely when skills are complementary/
additive; sparingly (and with selection) when entangled.

E10 (directed_sex) -- directed sex beats biological sex: biology is stuck
with 2 random-mating parents and no offspring preview; an AI can choose
complementary mates, evaluate many recombinant offspring, keep the fittest,
and use unbounded parents (iterated recombine-then-select). Random
("biological") sex craters with ruggedness (0.66->0.51); directed sex
tracks/exceeds the best parent at every ruggedness -- converting the
outbreeding-depression catastrophe into a win. No biological analog.

Complete sexual-transmission picture: dramatic super-parent offspring when
skills are complementary (E8); outbreeding-depression risk when entangled
(E9); directed sex resolves the risk (E10). configs/layer1/{E9,E10}.yaml,
figures/plot_{E9,E10}.py, READMEs, +5 tests (117 green).

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

results/E9/README.md

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+# E9 — landscape robustness: when recombination helps, and the outbreeding-depression risk
+
+**Claim tested.** E8 showed sexual recombination assembling super-parent offspring — but on an
+*additive* landscape, where recombination trivially helps. The honest, credibility-critical question
+(the classic "why sex?" problem): does the benefit survive **epistasis**, or does merging entangled
+models break them?
+
+**Setup.** Parents are **local optima** ("trained models") of a **Kauffman NK landscape** (`L=12`),
+whose ruggedness `K` (epistatic interactions per locus) is swept together with the **recombination
+rate**. `K=0` is additive/smooth; larger `K` is rugged (co-adapted allele blocks, many local optima).
+Fitness ∈ [0,1]. 24 replicate landscapes; 200 offspring per point.
+
+### Symbols
+- **NK landscape** — tunable-ruggedness fitness landscape; `K` = epistasis (0 = additive, high = rugged).
+- **recombination rate** — per-gap crossover probability (0 = clonal / copy a parent; 0.5 = free recombination, loci independent).
+- **outbreeding depression** — offspring *less* fit than parents because recombination broke co-adapted allele blocks.
+
+### The two panels
+1. **The risk.** Mean offspring fitness *minus* best parent, vs recombination rate, one curve per
+   ruggedness `K`. On the additive landscape (`K=0`) it's flat at 0; as `K` grows the curves plunge
+   **negative**, and deeper the higher the recombination rate — **outbreeding depression, worse the
+   more entangled the skills and the more you mix** (`K=8`, free recombination: ≈ −0.23).
+2. **With selection, an optimal rate re-emerges.** Best-of-brood fitness (offspring selection) vs
+   rate per `K`, with parents dotted. On rugged landscapes a **nonzero intermediate recombination
+   rate** is best — enough mixing to find new combinations, not so much that it shatters good blocks.
+
+### Takeaway
+Recombination is not a free lunch. **Merge freely when skills are complementary/additive; merge
+sparingly — and always *select* offspring — when they are entangled.** This is the celebrated
+population-genetics result (recombination load / outbreeding depression) reproduced for AI model
+merging, and it turns the sexual metaphor from a lucky demo into a law with a design rule. The rescue
+— directed sex with offspring selection — is E10. **Falsifier (not triggered):** if recombination
+rate had no effect, or free recombination never underperformed the parents on rugged landscapes, the
+epistasis caveat would be moot.