lily: add typing. fix old/invalid LilyPond output

.agents/skills/writing-docs/SKILL.md

@@ -0,0 +1,80 @@
+---
+name: writing-docs
+description: >-
+  House style for docstrings, code comments, and `Changed in`/`New in` version
+  markers in music21. Use whenever you write or edit a docstring or comment, add
+  a version marker, or decide where a bug fix's test belongs. Covers the length
+  target, the rule against narrating bugs you just fixed, and why regression
+  cases go in unittests rather than doctests.
+---
+
+# Writing docs and comments
+
+## Length
+
+Aim for about 40% of the length an LLM writes by default. Cut qualifiers,
+restatements, and the sentence that explains the sentence before it. A comment
+earning its place says something the code cannot.
+
+## Say what is, not what was or what not to do
+
+Describe current behavior. Do not narrate the bug you just fixed, the old
+spelling of an API, or when upstream changed something.
+
+```python
+# yes
+# ligature brackets; manual beams are plain [ and ]
+return self.backslash + '[ '
+
+# no
+# \[ used to be emitted for beams, which was wrong -- LilyPond renamed
+# this in 2.16 and it silently produced nothing
+```
+
+The commit message is where a fixed bug belongs: why it was wrong, how it was
+found, what it broke. That costs nothing until someone runs `git log`, and
+`git blame` leads them there from the line itself.
+
+The exception is a mistake that is likely to recur — a genuine trap that the
+next person would otherwise walk into. Rare. Prefer stating the rule positively
+even then.
+
+## Version markers
+
+`* Changed in v[X]: one line.` or `* New in v[X].` Only for user-facing changes
+to the public interface: a signature, a return type, an output format. Keep to
+what a reader must act on; drop the before-picture.
+
+```
+* Changed in v11: `stringOutput()` always returns a `str`, never None.
+* Changed in v11: emits `\tuplet`; the arguments are now actual, normal.
+* New in v11.
+```
+
+A plain bug fix — code now does what it always claimed — gets no marker and no
+doctest. It goes in the commit message.
+
+See the `bump-version` skill for which digit to change and the odd/even
+convention.
+
+## Doctests are not regression tests
+
+Doctests are documentation that happens to be verified. Every example must earn
+its place by teaching the reader something about how to use the object.
+
+Regression cases go in the module's `Test(unittest.TestCase)` class, where a
+comment or the method name can name the issue:
+
+```python
+def testMetronomeMarkWrittenInStream(self):
+    # https://github.com/cuthbertLab/music21/issues/1852
+    ...
+```
+
+So: a fix for a crash on an edge case, a check that some input no longer
+produces invalid output, an assertion tied to an issue number — unittest. An
+example a user would want to read — doctest.
+
+Naming the guarded bug **is** appropriate in a unittest; that is what the test
+is for. The rule against narrating old bugs applies to docstrings and to
+comments in shipping code, not to tests.