fix(billing-core): round decimal money exactly

[kang-heewon]

Summary

• Convert decimal money values to minor units with exact integer intermediates before applying the configured rounding mode, so binary floating-point multiplication cannot shift half-up boundaries.
• Preserve calculated decimals, repeating fractions, tiny values, and large amounts whenever the final minor-unit result remains a safe integer.
• Keep the public Money API unchanged and publish the behavior correction as a patch changeset for @croco/billing-core.

Closes #2472

Validation

• Validation profile: Standard
• Negative control: the focused regression failed on the prior implementation because 1.005 USD produced 100 instead of 101.
• Focused Money suite: 16/16 passed; full @croco/billing-core suite: 184/184 passed.
• @croco/billing-core lint, typecheck, and build passed.
• pnpm pr-review-companion -- --base origin/trunk --head HEAD --run-required-checks passed, including architecture policy, public API drift, and pnpm check.
• Normal pre-push gates passed with 243/243 full-repository test tasks and 243/243 guarded typecheck tasks.
• Independent exact-head code review passed with no findings after 50,352 oracle comparisons across currencies, rounding modes, IEEE-754 edge cases, subnormal values, signed zero, and safe-integer boundaries.

Integration

• Head: dd2f28b51133d77e6b27ecd3632c4dab5a1702d8
• Validated base: 3ae15edb806855391cb0a5698f1ff0a1d7ff4c01
• Current base: 00fe49487a3a5edfc736a58096fcdd6de1c20276
• Base drift is limited to audit-drizzle source, tests, documentation, and its changeset; it does not interact with the billing-core decimal conversion contract, and GitHub reports the PR conflict-free.
• No frontend-visible or UX-visible behavior changed.

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.

[coderabbitai]

Warning

Review limit reached

Next included review available in 38 minutes.

Check out review usage here.

View limit details

Limit details: You’ve used the included review currently available.

You've used all free OSS reviews for now. Wait for the free limit to reset to keep reviewing this public repository.

Learn how review limits work.

Review configuration:

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Advanced

Run ID: 21864b61-ee5a-4833-9dbb-871bb9084c3c

📥 Commits

Reviewing files that changed from the base of the PR and between 00fe494 and dd2f28b.

📒 Files selected for processing (3)

• .changeset/exact-money-decimal-rounding.md
• packages/billing-core/src/libs/Money.ts
• packages/billing-core/src/tests/Money.spec.ts

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands.}

[github-actions]

📊 Benchmark Results

✅ All benchmarks passed

Benchmark	p75	Threshold	Baseline	vs Baseline	Status	Notes
CrocoApp constructor	36.3μs	30.0ms	8.2μs	+343.5%	❌	-
CrocoApp lambdaHandler (10 controllers)	1.7ms	50.0ms	258.4μs	+574.3%	❌	-
Lambda cold-start simulation	1.5ms	80.0ms	418.1μs	+251.4%	❌	-
Lambda cold-start with headers	1.2ms	80.0ms	369.7μs	+224.7%	❌	-
Lambda cold-start with binary body	1.2ms	80.0ms	339.1μs	+253.2%	❌	-
Lambda cold-start with query params	1.2ms	80.0ms	301.3μs	+299.6%	❌	-
Lambda cold-start with authorizer context	1.2ms	80.0ms	299.8μs	+286.0%	❌	-
Lambda cold-start realistic scenario	1.1ms	80.0ms	299.2μs	+281.2%	❌	-
EventBusConfig.start (10 handlers)	1.7μs	10.0ms	1.4μs	+16.1%	✅	-
EventPublisher.publishNow single event	1.7μs	2.0ms	1.7μs	-0.7%	✅	-
DefaultHandlerResolver.resolve × 10	0.1μs	5.0ms	0.1μs	-25.0%	✅	-
Container.get singleton (cold)	52.5μs	5.0ms	70.3μs	-25.2%	✅	-
Container.register × 50 components	2.7ms	10.0ms	3.2ms	-15.9%	✅	-
Container.validate (50 components)	3.1ms	20.0ms	3.4ms	-7.5%	✅	-
Container.get singleton (warm)	1.2μs	500.0μs	1.6μs	-27.6%	✅	-
TelemetryRuntime.init (lambda preset)	9.9μs	200.0ms	1.1ms	-99.1%	✅	-
lambdaPreset config creation	1.1μs	2.0ms	1.4μs	-21.2%	✅	-

_{Updated: 2026-09-22T17:50:15.353Z · Commit: dd2f28b}