# Validate `SG_` signal definitions and report malformed or unsupported syntax

## Description

Following strict validation of `BO_` frame definitions, add equivalent validation for `SG_` signal definitions. Currently, some malformed statements are accepted silently, and unsupported syntax can produce incorrect decoding.

Examples of current behavior:

- `@2+` is interpreted as Motorola instead of being rejected.
- Numeric fields containing suffixes, such as `0junk` or `1junk`, are partially parsed and accepted.
- `SG_ Value m1 : ...` is treated as an ordinary signal named `Value m1`, without multiplexing conditions.
- A statement containing a tab after `SG_` is silently skipped.

## Scope

- Validate statement structure and require a nonempty signal name without whitespace.
- Accept spaces and tabs between syntax elements.
- Parse numeric fields completely, rejecting overflow and trailing characters; preserve support for fractional values and scientific notation in physical parameters.
- Accept only byte order `@0` or `@1` and signedness `+` or `-`.
- Require signal lengths within `1..64` bits.
- Validate that the signal fits within the frame's declared payload length, using the appropriate Intel or Motorola bit traversal.
- Reject NaN and infinite values for factor, offset, minimum, and maximum; require `minimum <= maximum`.
- Explicitly reject multiplexing markers `M`, `mN`, and their combinations until multiplexing is implemented.
- Preserve the error format `path:line: description`.

## Acceptance criteria

- [ ] Existing valid signal definitions continue to parse successfully.
- [ ] Malformed and unsupported definitions stop parsing with a clear error.
- [ ] Regression tests cover the cases above, numeric boundaries, and Intel/Motorola payload boundaries.
- [ ] Existing parser and decoder tests pass.
- [ ] README documents the new validation rules and limitations.

## Out of scope

Duplicate detection, multiplexing support, and `VAL_` support remain separate tasks.

## Suggested branch

`codex/validate-sg-definitions`