fix: high-precision fixed-point division (issue #346)

[mofejo1]

Closes #346

Summary:

• Add 7dp→7dp division helper using 10^9 internal scaling and rounded downscale to 10^7.
• Round when normalizing to 7 decimals to avoid truncation for low-value fractions.
• Add focused unit tests for low-value/rounding behavior.

Notes:

• Kept changes minimal and localized to math/helpers + small test compatibility fixes.

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