Add Chord.from_name, Interval constants, PROGRESSIONS, enharmonic, fix API docs

New features:
- Chord.from_name("Am7") — build chords from chart names
- Chord.__str__() — prints "C major" instead of raw tones
- Interval constants: Interval.PERFECT_FIFTH, MAJOR_THIRD, OCTAVE, etc.
- PROGRESSIONS dict: 8 common progressions as Roman numeral tuples
  ("I-V-vi-IV", "ii-V-I", "12-bar blues", etc.)
- Tone.enharmonic: C# → "Db", natural notes → None
- Key.__str__(): "C major"

Fix: docs CI now installs all dependencies (uv sync --group docs)
so autodoc can import pytheory modules. API reference pages were
blank on GitHub Pages because pytuning/scipy weren't installed.

New example: examples/explore.py — comprehensive demo of the full API.

393 tests.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.github/workflows/docs.yml

@@ -25,8 +25,11 @@ jobs:
       - name: Set up Python
         run: uv python install 3.13
 
+      - name: Install dependencies
+        run: uv sync --group docs
+
       - name: Build docs
-        run: uv run --group docs sphinx-build -b html docs docs/_build/html
+        run: uv run sphinx-build -b html docs docs/_build/html
 
       - name: Upload artifact
         uses: actions/upload-pages-artifact@v3