dashboard: split CV threshold — 5% joint / 2% constrained

Live-traffic diagnosis: typical single-session warm-cache load has the
same cached image across turns (pixel CV = 0%) AND text variance of
3-4% (verified 2026-05-19: 117k/127k/128k chars across three turns =
3.84% CV). Both columns failed the original 5%-everywhere gate, so the
constrained fallback I just shipped never actually activated — the
dashboard fell straight through to wandering stale constants.

The 5% gate exists to stop joint OLS from splitting two indistinguishable
parameters. In constrained mode β is pinned (no split to make), so we're
fitting one parameter through the origin — 1-D OLS is numerically stable
at much lower variance. 2% is conservative; even 1% would converge.

New thresholds:
  MIN_CV_JOINT       = 0.05  (unchanged — both columns must vary)
  MIN_CV_CONSTRAINED = 0.02  (NEW — text only, β-pinned through origin)

Real-world impact: live single-session traffic now activates constrained
mode after 3 compressed turns instead of waiting for cross-session
variance that may never arrive. Dashboard label flips from amber 'stale
constants' to blue 'constrained · α-only' — operator sees a measured α
within ~30 seconds of starting a fresh proxy.

Tests:
  + new: live-traffic-shape sample activates constrained mode (3.84% CV)
  + new: below-2% floor still rejects (no garbage 1-D fits)
  unchanged: 2-D collinear cases still null, joint-fit at high CV still
  returns mode='joint'

src/dashboard.ts

@@ -418,7 +418,17 @@ export class DashboardState {
     const varP = Math.max(0, syy / n - meanP * meanP);
     const cvX = meanX > 0 ? Math.sqrt(varX) / meanX : 0;
     const cvP = meanP > 0 ? Math.sqrt(varP) / meanP : 0;
-    const MIN_CV = 0.05;
+    // Joint mode threshold: 5% on BOTH columns. Below that, OLS can't tell
+    // whether token cost is coming from text or pixels and the α/β split
+    // wanders ±15 pp per sample (HANDOFF "What's blocking calibration").
+    const MIN_CV_JOINT = 0.05;
+    // Constrained mode threshold: 2% on TEXT only. β is pinned (no split
+    // to disambiguate), so we're fitting one parameter through the origin.
+    // 1-D OLS is numerically stable at much lower variance — typical live
+    // single-session traffic has 3-4% text CV (verified 2026-05-19 live:
+    // 117k/127k/128k chars over three turns = 3.84% CV). The 2% floor
+    // still rejects "literally identical text three times in a row".
+    const MIN_CV_CONSTRAINED = 0.02;
 
     // Anthropic's published per-pixel rate for the default image tiling:
     // 1 token per ~750 pixels (≈ 0.001333). Used by the constrained fallback
@@ -427,7 +437,7 @@ export class DashboardState {
     const ANTHROPIC_BETA = 1 / 750;
 
     // ---- Mode 1: joint OLS (both columns vary) ----
-    if (cvX >= MIN_CV && cvP >= MIN_CV) {
+    if (cvX >= MIN_CV_JOINT && cvP >= MIN_CV_JOINT) {
       const det = sxx * syy - sxy * sxy;
       if (det !== 0) {
         const alpha = (syy * sxt - sxy * syt) / det;
@@ -462,7 +472,7 @@ export class DashboardState {
     //
     // (derivation: minimize Σ(α·x − r)² → dL/dα = 0 → α = Σxr / Σx²
     //  where r_i = tokens_i − β · pixels_i, so Σx·r = sxt − β·sxy.)
-    if (cvX < MIN_CV || sxx === 0) return null;
+    if (cvX < MIN_CV_CONSTRAINED || sxx === 0) return null;
     const alphaConstrained = (sxt - ANTHROPIC_BETA * sxy) / sxx;
     if (alphaConstrained <= 0) return null;
     return {

tests/dashboard-fit.test.ts

@@ -101,13 +101,41 @@ describe('DashboardState.fitCosts() — empirical α/β regression', () => {
 
   it('returns null when text_chars column is constant', () => {
     // Mirror case — same body shape across samples, only cache state varies.
-    // Without text variance, α is unidentifiable.
+    // Without text variance, α is unidentifiable in EITHER joint or
+    // constrained mode (constrained still needs text to vary).
     dash.update(ev({ textChars: 130_000, pixels: 21_000_000, input: 5, cacheCreate: 500, cacheRead: 141_680 }));
     dash.update(ev({ textChars: 130_000, pixels: 23_000_000, input: 5, cacheCreate: 300, cacheRead: 142_447 }));
     dash.update(ev({ textChars: 130_000, pixels: 25_000_000, input: 5, cacheCreate: 200, cacheRead: 143_119 }));
     expect(dash.fitCosts()).toBeNull();
   });
 
+  it('constrained-mode CV threshold (2%) accommodates real single-session text variance', () => {
+    // Live single-session-warm-cache traffic from 2026-05-19: three turns
+    // hit the proxy with text chars 117,927 / 127,512 / 128,588 and the
+    // SAME cached image (pixels constant at 22,792,816). Text CV = 3.84%.
+    // Under the original joint-mode-only 5% gate this returned null and the
+    // dashboard fell back to wandering stale constants. The split-threshold
+    // version activates constrained mode here (β pinned, α measured) so
+    // the operator gets a measured-α answer instead of stale constants.
+    dash.update(ev({ textChars: 117_927, pixels: 22_792_816, input: 6, cacheCreate: 1116, cacheRead: 124_236 }));
+    dash.update(ev({ textChars: 127_512, pixels: 22_792_816, input: 476, cacheCreate: 6103, cacheRead: 125_352 }));
+    dash.update(ev({ textChars: 128_588, pixels: 22_792_816, input: 1, cacheCreate: 2432, cacheRead: 131_455 }));
+    const fit = dash.fitCosts();
+    expect(fit).not.toBeNull();
+    expect(fit!.mode).toBe('constrained');
+    expect(fit!.alpha).toBeGreaterThan(0);
+  });
+
+  it('still rejects below the 2% floor (literally-identical-text case)', () => {
+    // Below 2% text CV the residual signal is too thin to measure α even
+    // with β pinned. Make sure we still return null instead of producing a
+    // garbage 1-D fit.
+    dash.update(ev({ textChars: 130_000, pixels: 22_792_816, input: 6, cacheCreate: 1116, cacheRead: 124_236 }));
+    dash.update(ev({ textChars: 130_500, pixels: 22_792_816, input: 6, cacheCreate: 1116, cacheRead: 124_236 }));
+    dash.update(ev({ textChars: 130_800, pixels: 22_792_816, input: 6, cacheCreate: 1116, cacheRead: 124_236 }));
+    expect(dash.fitCosts()).toBeNull();
+  });
+
   it('returns mode="joint" when both columns vary (full empirical fit)', () => {
     // Both columns vary > 5% — joint OLS is identifiable. mode tags the
     // headline number as fully empirical so the operator can distinguish