wc-population-diversity — anti-inbreeding & selection-pressure governor

Implements the diversity-maintenance and selection-pressure ideas the Evolution document is explicit about: "plant breeders care about preserving genetic diversity; agent researchers care about maintaining behavioural diversity — same problem," and "selection pressure too high → population collapses; too low → progress is slow," and "repeatedly refining the same strategy leads to local optima" (inbreeding). In this system, diversity isn't an aesthetic — it's what guarantees the manager sees a genuine choice (a cover path and a climb path), not six shades of the same template.

Inputs

• The offspring set (post-recombination, post-mutation), each with squad/plan, ownership profile, and variance_band.
• The objective θ/k and the elite-set history (was selection already monochrome upstream?).

Workflow

- [ ] 1. Measure collapse metrics across the offspring set
- [ ] 2. Compare against diversity floors
- [ ] 3. If collapsed: inject an under-represented genotype and/or raise mutation; signal re-run of crossover+mutation
- [ ] 4. Tune selection pressure (widen/tighten the elite set)
- [ ] 5. Emit the diversity report

Step 1: Collapse metrics

• Squad overlap: mean pairwise % of shared players across offspring. >~75% mean overlap = collapsed.
• Captain overlap: do all offspring captain the same player / same day? Total captain overlap = collapsed (captaincy is the highest-leverage axis; it must vary).
• Ownership-profile spread: the spread of offspring across the template↔differential axis (mean effective-ownership of each offspring's distinctive picks). All clustered at "template" or all at "differential" = collapsed on the rank axis.
• Variance-band coverage: does the set include ≥1 low-variance AND ≥1 high-variance offspring? Missing a band = collapsed on the selection-pressure axis.

Step 2: Diversity floors (the board must clear these)

A healthy offspring set, before it reaches the manager, must satisfy:

• mean pairwise squad overlap < ~75%,
• at least 2 distinct captain choices (or captain-ladder shapes) across the set,
• variance-band coverage: ≥1 low and ≥1 high band present,
• ownership spread: at least one cover-leaning and one differential-leaning option.

These exist so decision-board-format.md's "2–4 genuinely distinct options spanning the variance spectrum" is structurally guaranteed, not hoped for.

Step 3: If collapsed — re-seed wider, mutate, re-run

The Evolution document's remedies plus the systems-thinking digest's broad-band respawn (system-dynamics.md §1–2):

• **Re-seed from a wider kernel, not the mean. Identify the under-represented corner (usually high-variance / differential when selection pressure was high) and force that archetype's blocks in wider than the current population's spread** — if everything converged to chalk, inject an A2/A4 rebuild braver than any survivor, not a copy of the survivors' average. Averaging toward the collapsed centre is the wrong move; widen away from it. (Migration with a wide kernel — the digest's "respawn from a distribution wider than the current std".)
• Raise the mutation rate for the re-run so the regenerated offspring explore further from the collapsed template.
• Signal the engine to re-run crossover (Phase 3) + mutation (Phase 4) for the affected offspring, then re-measure. Cap at 2 re-runs to avoid loops; if still collapsed after 2, emit what you have with an explicit "low-diversity board" flag so the Director can tell the manager the options are genuinely close this round (sometimes one template really is dominant — that's honest, but say so).

Step 3.5: Protected-invariant guard + critical-numerosity probe

• Protected-invariant guard (invariants.md §4, §10). This guard is itself invariant: no learning-loop update may weaken it, and no archetype is ever zeroed. Before accepting any soft-prior tilt the scoreboard proposes, confirm it would not (a) drive a genotype's weight to zero, (b) let one genotype's blocks dominate every offspring, or (c) disable/soften this diversity check. If it would, reject the tilt and report it — the population search has started eating its own safety rail (the digest's #1 failure: premature convergence + the self-update overwriting the diversity monitor).
• Critical-numerosity probe (system-dynamics.md §5). Population size N (the archetype count) is not monotonic. If the same decision rerun keeps producing qualitatively different winners — high run-to-run instability in which option leads — that's the signature of sitting near a critical N. Flag it to the Director to probe N−1 and N+1 rather than reflexively adding lanes; prefer the count that yields a stable, spectrum-spanning board. More archetypes is not automatically better.

Step 4: Selection-pressure tuning

• Too high (every stage kept only the single best fitness → monochrome set): widen the elite set, force the variance spread back in. Pressure that collapses the gene pool defeats the population.
• Too low (offspring are noise, no quality concentration): tighten — narrow toward the higher-fitness fusions. The target is a healthy middle: 2–4 offspring that are distinct and good.
• Pressure direction also tracks θ — protect legitimately tilts the recommended default toward low-variance, but it must not delete the high-variance option from the board (re-weight, don't collapse — fitness-function.md).

Step 5: Emit the diversity report

diversity_report:
  mean_squad_overlap: <%>
  distinct_captains: <n>
  variance_bands_present: [low, medium, high?]
  ownership_spread: cover↔differential coverage: ok|gap
  collapsed: false|true
  actions_taken: [ "injected A2 differential pod into offspring_3", "raised mutation 0.1→0.25", "re-ran crossover ×1" ]
  board_flag: none | "low-diversity: options genuinely close this round"

This report goes to the Director so the manager knows whether the board is a wide-open choice or a narrow one. Transparency about diversity is part of the advisory contract.

Guardrails

• The board must clear the diversity floors or carry an explicit low-diversity flag. Never present six near-identical squads as a "choice."
• Re-weight, don't delete. θ tilts the default; it never removes the opposite-variance option.
• Cap re-runs at 2. Sometimes convergence is correct — say so rather than thrash.