Add human-readable messages for FloatError WasmEncodedError

[thedavidmeister]

Fixes the // todo: add detailed readable msg for errors in crates/float/src/error.rs: WasmEncodedError.readable_msg previously just duplicated the short msg (a Display/Debug dump).

What changed

• FloatError::to_readable_msg(&self) -> String — a match over every FloatError variant producing a detailed, human-readable message (following the established rain ecosystem pattern, e.g. raindex's Error::to_readable_msg).
• DecimalFloatErrorSelector::to_readable_msg(&self) -> &'static str — a detailed message per Solidity DecimalFloat error selector (all 9 variants: CoefficientOverflow, ExponentOverflow, ExponentUnderflow, FixedDecimalOverflow, Log10Negative, Log10Zero, LossyConversionFromFloat, NegativeFixedDecimalConversion, WithTargetExponentOverflow), derived from the @dev docs of the underlying error/*.sol definitions.
• From<FloatError> for WasmEncodedError now sets readable_msg from to_readable_msg() (it kept msg = to_string() for the short message).

This is mechanical message text only — no change to any error condition, selector mapping, or control flow.

Tests

Added a mutation-validated #[cfg(test)] mod tests in error.rs (source-organized) asserting the exact message for representative variants, the known/unknown DecimalFloatSelector arms, and that WasmEncodedError wires readable_msg from to_readable_msg (distinct from msg).

Mutation-validated: breaking the Log10Zero selector text killed both selector-asserting tests; breaking the readable_msg wiring (back to to_string()) killed the WasmEncodedError test; restored to green.

cargo test -p rain-math-float green (82 lib unittests + 36 doctests), cargo fmt --check and cargo clippy --all-targets clean.

Scoped entirely to crates/float/src/error.rs; does not touch canonicalize (#237) or pow (#232/#239) work.

Closes #77

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📥 Commits

Reviewing files that changed from the base of the PR and between 95108e2 and 4e770aa.

📒 Files selected for processing (1)

• crates/float/src/error.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch 2026-06-14-issue-77

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