fix: round calibration deltas for display

[Dibya0912]

Summary

Calibration deltas such as -4/7 currently display all floating-point digits. Show at most two decimal places using the same en-US number-formatting convention as the balance card, with the original value available in the title attribute.

Related Issues and Pull Requests

Fixes #65

Changes

• Format signed means, medians and the difference between means with at most two fractional digits.
• Display zero when rounding would otherwise produce +0 or minus zero.
• Preserve raw values in title attributes and add a rendering regression test.

Testing

Using the requested Node 24.17.0 and pnpm 10.33.0: both calibration test files pass (12 tests); pnpm typecheck passes; ESLint passes on both changed files. git diff --check passes.

Footer

Generated by GPT-6 (implementation, testing, review)

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[Nitjsefnie]

The rounding itself is correct — it even fixes a −0 case that balance-card.tsx still gets wrong. Four things before merge:

1. The test doesn't pin the rounding. toHaveTextContent is a substring match, so −0.5714285714285714 satisfies "−0.57". With the rounding stripped out and the titles left in, the suite still passes 3/3. Anchor lines 19–20 (/^Mean delta −0\.57$/) and add one value that rounds up, e.g. -0.999 → −1.

2. title on a <p> has no touch or keyboard trigger and is read aloud in full by screen readers. A visible <span className="mono-meta"> gets you the same thing, copyable.

3. The sign rule diverges from balance-card.tsx, which issue #65 named as the model — yours decides from the rounded string, it decides from value. Yours is better; just call it out as deliberate.

4. d6ded91 is missing its trailer. Please amend with Co-Authored-By: GPT-6 Astra <noreply@openai.com> (space, not hyphen).

Also: comment /claim on issue #65 — the assignee is what reserves your credit for this, and nothing is assigned yet.

[Nitjsefnie]

Closing as stale and unmaintained, plus once #55 lands, your PR would be autoclosed under the rules.