fix(time-signature): return NONE instead of throwing on non-matching input

[youdie006]

Fixes #490

Problem

TimeSignature.get / parse throw TypeError: Cannot read properties of undefined (reading 'split') on any string that doesn't match the <n>/<n> regex -- most notably "C" (common time), but also "", "x", etc.

Root cause

In parse, const [_, up, low] = REGEX.exec(literal) || [] yields an undefined up when the regex doesn't match. That recurses into the array branch where up.split("+") (index.ts:72) throws.

Fix

Add a defensive guard: when up is not a string (regex miss), return the zeroed parse [0, 0], which the existing build() already maps to the NONE / empty signature -- the same path invalid inputs like "0/0" already take. No behavior change for valid input.

Tests

Added a regression test in packages/time-signature/test.ts asserting that "C", "", "x", "4", "/", "abc" do not throw and return the empty signature, and that "4/4" still parses correctly.

• Reverting the fix -> the new test reproduces the exact Cannot read properties of undefined (reading 'split') error (red).
• With the fix -> package tests 8/8 pass; full suite 283/283 pass; lint and tsdown build clean (green).

---

This contribution was prepared with AI assistance.