Support sus4 chords in root-finding algorithm

[p2rez]

Fixes #1650.

Note: This contribution was developed with AI assistance (Claude/Anthropic), as declared per the contributing guidelines.

Problem

chord.Chord('F B- C E-').root() incorrectly returned C instead of F. The _findRoot method's rootnessFunction only scored notes based on thirds, fifths, sevenths, etc. above them, but gave no credit for a fourth, so sus4 chord roots were never correctly identified.

Solution

Added sus4 chord detection to _findRoot. Before scoring, we check whether any note in the chord has both a third and fifth above it. If none do (indicating a sus4 chord), we apply a bonus to notes that have both a fourth and fifth above them, giving sus4 roots the same base score as notes with a third above them in regular chords.

This avoids incorrectly boosting notes in chords that happen to have an eleventh (like C11 = C E G B- D F), where C has both a third and fifth above it, so the sus4 path is not triggered.

Tests

Added testSus4RootFinding in test/test_chord.py covering:

Basic sus4 seventh chord from the issue (F B- C E- → F)
Simple sus4 triad (C F G → C)
sus4 with seventh (C F G B- → C)
Regression: regular major, minor, dominant seventh still correct
Regression: C11 chord still returns C, not F

All 27 tests pass.

[mscuthbert]

tests do not cover more difficult cases. unwieldy 2-part sol (thanks for AI attempt)