fix: preserve sign in truncate_float (negatives silently became 0.0)

[PrazwalR]

Problem

truncate_float (public API, src/helpers.rs) casts (float * pow10) to
u64. In Rust, casting a negative f64 to an unsigned int saturates to 0,
so every negative input is silently destroyed:

truncate_float(-123.456, 2, false) // returns 0.0, expected -123.45
This is a pub function in a trading SDK used for price/size math, so any
downstream caller passing a negative value (PnL, margin, deltas) gets a silently
wrong result.

Fix
Truncate on the magnitude and reapply the sign:

let sign = float.signum();
let mut abs_truncated = (float.abs() * pow10) as u64;
if round_up { abs_truncated += 1; }
sign * abs_truncated as f64 / pow10
Positive path: byte-identical to previous behavior.
round_up now grows magnitude (away from zero) for both signs.
NaN propagates instead of silently returning 0.0.
Tests
Added 6 tests in helpers::tests:

concrete negatives, positive-path parity, round_up both signs, 0.0/-0.0,
NaN propagation
antisymmetry property test: truncate(-x) == -truncate(x) across 2000
magnitudes × 9 decimals × 2 flags (36k assertions)
cargo test --lib          # 18 passed (12 existing + 6 new)
cargo clippy --all-targets -- -D warnings   # clean
cargo fmt -- --check       # clean
Scope note
There is a separate, pre-existing float-truncation issue (0.29 → 0.28)
present in both the old and new code on the positive path. It is unrelated to the
sign bug and left untouched to keep this diff minimal and signing-hash-safe.

## Notes
- Branch name assumes upstream default `master`. If your fork tracks a different upstream, point the PR base accordingly.
- `git switch -c` creates a branch — you asked me not to, so **you** run it. I created/committed nothing.
- Kept diff to one file. Issues 2/4/5 stay unbundled for their own PRs.

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