fix: guard mul() exponent-sum overflow, surface as ExponentOverflow

[thedavidmeister]

Summary

• pow with very large or very negative integer exponents uses exponentiation-by-squaring, which repeatedly squares the base. When the base exponent is extreme, repeated doubling drives it past int256 bounds, causing a checked addition in mul() to panic with Panic(0x11) instead of the expected ExponentOverflow.
• Fixed by adding an exact overflow guard to mul(): checks that exponentA + exponentB itself would overflow before performing the addition. The previous approach (checking each operand against EXPONENT_MAX individually) was too broad — opposite-sign pairs can never overflow int256, but the div round-trip tests pass exponents of opposite signs (one near type(int256).max, the other type(int256).min) whose sum is safely near zero.
• New test testPowNegativeExponentSquaringPanic pins the exact revert args for the negative-exponent squaring-loop case.
• Updated deploy constants and ABI artifact for the implementation bytecode change. Deploy needs manual trigger before testProdDeployment* will pass: gh workflow run manual-sol-artifacts.yaml --repo rainlanguage/rain.math.float --ref 2026-06-17-issue-239-pow-panic-exponent-overflow -f suite=decimal-float

Closes #239

Test plan

• testPowNegativeExponentSquaringPanic passes (ExponentOverflow instead of Panic)
• testPowIntegerExponentSquaringOverflow still passes (unchanged error path)
• All div round-trip tests pass (testDivAdjustExponent*)
• testArtifactsCommitted passes
• testDeployAddress / testExpectedCodeHashDecimalFloat pass
• Full fuzz suite passes (testRoundTripFuzzPow)
• Deploy triggered manually → testProdDeployment* passes

🤖 Generated with Claude Code

Co-Authored-By: Claude noreply@anthropic.com

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Review details

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro

Run ID: 3545ae4d-e3b7-4621-8f8f-88106eff54bc

📥 Commits

Reviewing files that changed from the base of the PR and between 774ed07 and 312d427.

📒 Files selected for processing (4)

• crates/float/abi/DecimalFloat.json
• src/lib/deploy/LibDecimalFloatDeploy.sol
• src/lib/implementation/LibDecimalFloatImplementation.sol
• test/src/lib/LibDecimalFloat.pow.t.sol

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch 2026-06-17-issue-239-pow-panic-exponent-overflow

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