feat(gate): DescriptionLengthGate — MDL promotion gate (sibling to HeldOutGate)

[drewstone]

What

Adds DescriptionLengthGate — a Minimum-Description-Length promotion gate, sibling to HeldOutGate, in src/description-length-gate.ts. Implements the Builder/Breaker acceptance rule from Wang & Buehler, Self-Revising Discovery Systems for Science (arXiv:2606.01444, MIT LAMM):

L(M, D) = λ·L_model(M) + L_data(D | M)
accept M' over M  iff  L(M', D) < L(M, D)

• L_model = gzip-bit size of the candidate's model text (a deterministic Kolmogorov-complexity proxy).
• L_data = residual surprise −Σ log2(score) over the shared tasks, capped at a score floor (a failed task costs ~10 bits, never ∞).
• Both candidate and baseline are scored on the same enlarged evidence (every shared task, not just the held-out split).

Why merge it

HeldOutGate answers "does the candidate generalize on held-out items?" via a paired-delta CI + an overfit-gap heuristic. DescriptionLengthGate answers a different, complementary question: "does the candidate explain the evidence more cheaply?" — it charges the candidate for its own size, so a model that scores better only by bloating (memorizing counterexamples) grows L_model faster than it shrinks L_data and is rejected with rejectionCode: 'model_bloat'. That's the principled, information-theoretic form of the overfit penalty HeldOutGate approximates — useful whenever the model text whose size you want to penalize is available (a prompt, skill, profile, or symbolic model). It needs no held-out split, which makes it usable in self-improvement loops where every task is accumulated evidence.

It's a pure, additive primitive: a substrate gate alongside the existing one, callers opt in by constructing it.

Notes

• Zero new dependencies (node:zlib); no upward deps — substrate-pure.
• Pure & deterministic; 9 unit tests incl. the anti-overfit core (a higher-scoring but bloated candidate is rejected) and the calibration property (a larger model promotes only once accumulated evidence pays for its bits; lambda is the lever).
• Full suite green locally: 1913 passed, biome + typecheck + build + verify:package all clean.