Platoon polarity: VERIFIED correct, pinned by a standing test

Report-only verification of the two unproven claims from the Session-77 wiring,
which fired platoon on synthetic split-less hitters where the K=600 regression
zeroed the multiplier and masked both direction and resolution. No product code
changed — this adds one standing regression test.

FIXTURE — Yordan Alvarez (LHB), real 2026 splits, deep both hands: 115 PA vs LHP
at .529 slg, 327 PA vs RHP at .695, overall .652. The regression leaves a
material multiplier both ways (0.97 vs LHP, 1.023 vs RHP), so unlike the last
test this fixture can actually reveal direction.

RESOLUTION — verified on the live slate that the pitcher-hand attached to a
hitter is the OPPOSING team's probable, not his own. CLE (home) resolved to
Minnesota's away starter 696070; MIN (away) resolved to Cleveland's home starter
800048. The chain — hitter's team, the game, the other team, that team's
probable, that pitcher's hand — is correct, and it is pinned independently of
direction because a backwards resolution is invisible on a neutral hitter.

DIRECTION — deterministic L-vs-R on the frozen Alvarez fixture. Facing RHP nudges
UP (1.023) because he slugs .695 there, above his .652 overall — a favorable
opposite-hand matchup, exactly what platoon theory predicts for a left-handed
bat. Facing LHP nudges DOWN (0.97). The two move opposite directions, and
crucially the SPECIFIC sides are asserted, not merely "opposite" — a
mirrored-but-inverted implementation would put RHP below 1 and fails here. Both-
backwards is ruled out.

The test also pins a REVERSE-split hitter, Brandon Nimmo, who hits better vs LHP
than RHP. His multiplier goes up vs LHP, following his real numbers rather than a
hardcoded LHB-vs-RHP assumption — proof the sign is data-driven, which is the
correct design.

VERDICT: PASS. Resolution correct, direction correctly signed against both the
real split and platoon theory. Pinned by tests/unit/platoonPolarity.test.js so
the polarity cannot silently regress — the opp_rank_stat lesson applied.

Tests 3750 passed / 302 suites.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VCNgGSt5qvcLxaeQqa7Zpj

tests/unit/platoonPolarity.test.js

@@ -0,0 +1,110 @@
+/* ============================================================
+   Session 78 — PLATOON POLARITY LOCK (standing regression).
+
+   The opp_rank_stat lesson: polarity needs a PERMANENT assertion, not a
+   one-time eyeball. A backwards opposing-pitcher resolution OR a backwards
+   multiplier sign both look fine on a near-neutral hitter — so this pins BOTH
+   on a real, materially-split hitter (Yordan Alvarez, LHB, 2026), with frozen
+   numbers so it can never silently drift.
+
+   If this test ever fails, platoon has been broken in one of two ways and the
+   Session-77 wiring can no longer be trusted — do not "fix the test", find the
+   regression.
+   ============================================================ */
+
+const pl = require('../../src/services/platoonSplits');
+const ec = require('../../src/services/environmentContext');
+
+// Yordan Alvarez, real 2026 splits — deep both hands, textbook LHB platoon.
+// vs RHP he slugs far above his overall (advantage); vs LHP below (disadvantage).
+const ALVAREZ = {
+  vsL: { pa: 115, slg: 0.529, avg: 0.250 },
+  vsR: { pa: 327, slg: 0.695, avg: 0.290 },
+  overall: { pa: 442, slg: 0.652, avg: 0.278 },
+};
+
+describe('DIRECTION — signed against the real split AND platoon theory', () => {
+  const est = (ph) => pl.platoonEstimate({ splits: ALVAREZ, batterHand: 'L', pitcherHand: ph, statType: 'total_bases' });
+
+  it('produces a MATERIAL multiplier both ways (fixture can reveal direction)', () => {
+    // A neutral subject would reproduce the inconclusive Session-77 test.
+    expect(est('L').material).toBe(true);
+    expect(est('R').material).toBe(true);
+  });
+
+  it('vs RHP nudges UP — favorable split (opposite hand) → over more likely', () => {
+    // .695 slg vs RHP > .652 overall → favorable → UP. Backwards would be < 1.
+    expect(est('R').multiplier).toBeGreaterThan(1);
+  });
+
+  it('vs LHP nudges DOWN — unfavorable split (same hand)', () => {
+    // .529 slg vs LHP < .652 overall → unfavorable → DOWN. Backwards would be > 1.
+    expect(est('L').multiplier).toBeLessThan(1);
+  });
+
+  it('the two hands move OPPOSITE directions', () => {
+    const l = est('L').multiplier;
+    const r = est('R').multiplier;
+    expect((l < 1)).not.toBe((r < 1));
+  });
+
+  it('BOTH-BACKWARDS is ruled out — a mirrored-but-wrong sign fails here', () => {
+    // If the whole thing were inverted, RHP would be < 1 and LHP > 1. Assert the
+    // exact expected side, not merely "opposite".
+    expect(est('R').multiplier).toBeGreaterThan(1); // must be the RHP side that's up
+    expect(est('L').multiplier).toBeLessThan(1);    // and the LHP side that's down
+  });
+
+  it('a REVERSE-split hitter follows his real numbers, not the platoon prior', () => {
+    // Brandon Nimmo (LHB) hits BETTER vs LHP than RHP — unusual. The estimate
+    // must follow HIS data (up vs LHP), proving the sign is data-driven and not
+    // a hardcoded LHB-vs-RHP assumption.
+    const nimmo = { vsL: { pa: 104, slg: 0.510 }, vsR: { pa: 301, slg: 0.405 }, overall: { pa: 405, slg: 0.432 } };
+    const vsL = pl.platoonEstimate({ splits: nimmo, batterHand: 'L', pitcherHand: 'L', statType: 'total_bases' });
+    const vsR = pl.platoonEstimate({ splits: nimmo, batterHand: 'L', pitcherHand: 'R', statType: 'total_bases' });
+    expect(vsL.multiplier).toBeGreaterThan(1); // favorable vs LHP per HIS split
+    expect(vsR.multiplier).toBeLessThan(1);
+  });
+});
+
+describe('RESOLUTION — the OPPOSING team\'s pitcher, never the hitter\'s own', () => {
+  // A backwards resolution runs platoon on the wrong hand — invisible on a
+  // neutral hitter, so it is pinned independently of direction.
+  const SCHEDULE = { games: [
+    { homeTeam: { name: 'Cleveland Guardians' }, awayTeam: { name: 'Minnesota Twins' }, gameTime: '2026-07-21T22:40Z', venue: 'Progressive Field' },
+  ] };
+  const PITCHERS = { games: [
+    { home: { team: 'Cleveland Guardians', pitcherId: 800048 }, away: { team: 'Minnesota Twins', pitcherId: 696070 } },
+  ] };
+  const PEOPLE = { people: [
+    { id: 800048, pitchHand: { code: 'L' } }, // CLE's own SP
+    { id: 696070, pitchHand: { code: 'R' } }, // MIN's own SP
+  ] };
+  const fetchJson = async (url) => {
+    if (url.includes('/pitchers')) return PITCHERS;
+    if (url.includes('personIds=')) return PEOPLE;
+    if (url.includes('open-meteo')) return { hourly: { time: [], temperature_2m: [] } };
+    if (url.includes('statSplits')) return { stats: [] };
+    return SCHEDULE;
+  };
+
+  it('a HOME hitter faces the AWAY team\'s pitcher hand', async () => {
+    const ctx = await ec.buildContext('mlb', { fetchJson, schedule: SCHEDULE, pitchers: PITCHERS, people: PEOPLE });
+    // CLE hitter should resolve to MIN's pitcher (696070, R), NOT CLE's own (800048, L).
+    const oppPid = ctx._internals.oppPitcherByTeam.get('CLE');
+    expect(oppPid).toBe(696070);
+    expect(ctx._internals.handById.get(oppPid)).toBe('R');
+  });
+
+  it('an AWAY hitter faces the HOME team\'s pitcher hand', async () => {
+    const ctx = await ec.buildContext('mlb', { fetchJson, schedule: SCHEDULE, pitchers: PITCHERS, people: PEOPLE });
+    const oppPid = ctx._internals.oppPitcherByTeam.get('MIN');
+    expect(oppPid).toBe(800048); // CLE's SP, not MIN's own
+    expect(ctx._internals.handById.get(oppPid)).toBe('L');
+  });
+
+  it('the two teams get DIFFERENT pitchers — never the same (own) arm', async () => {
+    const ctx = await ec.buildContext('mlb', { fetchJson, schedule: SCHEDULE, pitchers: PITCHERS, people: PEOPLE });
+    expect(ctx._internals.oppPitcherByTeam.get('CLE')).not.toBe(ctx._internals.oppPitcherByTeam.get('MIN'));
+  });
+});