Fix panic in format64_to_fixed when rounding carries past all integer digits

[Ansh-699]

Summary

Fixes #55.

format64_to_fixed panics in debug builds when rounding causes a carry that propagates through all integer digits (e.g. 9.5 with fraction_digits=0).

Root Cause

The rounding loop calls result.get(round_index) before checking round_index == -1, causing an out-of-bounds read when every integer digit is 9 and the carry reaches index -1.

Fix

Move the round_index == -1 boundary check before the result.get() call, and split it from the c == b'-' check into its own branch.

Tests

Added rounding_carry_past_all_digits and rounding_carry_negative tests. All 70 tests pass.