chore(ab-corpus): add prompt-size measurement helper

One-shot diagnostic that prints char + token-estimate sizes for every
(variant, difficulty) combination of buildSystemPrompt. Used to verify
the elements-cookbook diet is shaving the bytes the test floor
predicts (>10kb) before running an actual sweep. Confirms today's
numbers: B 15kb, T-obvious 57kb, T-composite 75kb — diet saves ~17kb
(~4.4k tokens) on every obvious prompt. Phase 2 (per-domain split)
still needed to hit the 6-8k target.

scripts/ab-corpus/measure-prompt-sizes.ts

@@ -0,0 +1,35 @@
+#!/usr/bin/env bun
+/**
+ * Quick inline diagnostic — prints character + 4:1-token-estimate sizes
+ * for every (variant, difficulty) combination of `buildSystemPrompt`.
+ * Run when tuning the elements-cookbook diet to confirm the gate is
+ * actually shaving the bytes the test floor predicts:
+ *
+ *   bun scripts/ab-corpus/measure-prompt-sizes.ts
+ *
+ * Not wired into the harness — purely for human inspection. The
+ * authoritative regression guard is build-prompt.test.ts (>10kb floor).
+ */
+
+import { buildSystemPrompt } from './build-prompt';
+
+const cases: Array<{ label: string; variant: 'B' | 'T'; difficulty?: 'obvious' | 'optional' | 'composite' }> = [
+  { label: 'B (any difficulty)                ', variant: 'B' },
+  { label: 'T + difficulty="obvious"          ', variant: 'T', difficulty: 'obvious' },
+  { label: 'T + difficulty="optional"         ', variant: 'T', difficulty: 'optional' },
+  { label: 'T + difficulty="composite"        ', variant: 'T', difficulty: 'composite' },
+  { label: 'T + (no difficulty, safe default) ', variant: 'T' },
+];
+
+const tObvious = buildSystemPrompt('T', { difficulty: 'obvious' }).system.length;
+const tComposite = buildSystemPrompt('T', { difficulty: 'composite' }).system.length;
+const savings = tComposite - tObvious;
+
+process.stdout.write(`Variant / Difficulty | Chars   | ~Tokens (chars/4)\n`);
+process.stdout.write(`---------------------+---------+------------------\n`);
+for (const c of cases) {
+  const built = buildSystemPrompt(c.variant, c.difficulty ? { difficulty: c.difficulty } : {});
+  const len = built.system.length;
+  process.stdout.write(`${c.label} | ${String(len).padStart(7)} | ${String(Math.round(len / 4)).padStart(6)}\n`);
+}
+process.stdout.write(`\nObvious-vs-composite savings: ${savings} chars (~${Math.round(savings / 4)} tokens)\n`);