feat(design-systems): central prose — guidance on the system, rationale on tokens

Last piece of the architecture in #2296. The operator: "the prose doesn't have
to live as one offs, there's a central system for managing it."

Two fields, both free-form:

  design_systems.guidance   the narrative a token table cannot hold — aesthetic,
                            voice and tone, what is deliberately out of scope.
  design_tokens.rationale   WHY a token is this value, which is a different
                            question from `purpose` (what it is FOR). "Success
                            equals Moss, aligned by design" is a rationale;
                            "page bg, deepest surface" is a purpose. Rules carry
                            the first routinely and a token row had nowhere to
                            put it.

Free-form rather than a column per category, deliberately. A schema with
`voice`, `aesthetic` and `scope` columns would bake one rulebook's table of
contents into every install (rule #115), leaving the next install three empty
columns and nowhere for what it actually cares about. Both nullable: a design
system with no prose at all is complete, not a draft.

`rationale` cascades like `purpose` — deepest non-empty wins — so an app
overriding a colour keeps the family's reasoning rather than blanking it. Same
argument as `supersedes`: the override was about the value, not the meaning.

In the generated sheet the inline comment prefers `purpose` and falls back to
`rationale`, so a token carrying only the why still says something instead of
rendering bare.

tests/test_design_stylesheet.py

@@ -319,3 +319,24 @@ def test_clean_code_reports_nothing_to_act_on():
     assert report["superseded_literals"] == []
     assert report["local_definitions"] == []
     assert report["used"] == ["--fs-obsidian", "--fs-parchment"]
+
+
+def test_the_inline_comment_falls_back_to_rationale_when_there_is_no_purpose():
+    """A token carrying only the why still says something in the sheet, rather
+    than rendering bare because the other field happened to be empty."""
+    token = SimpleNamespace(
+        name="--fs-success", value_by_mode={"base": "#4a5d3f"},
+        group_name=None, purpose=None, rationale="equals Moss, by design",
+    )
+    assert "equals Moss, by design" in render_stylesheet([token])
+
+
+def test_purpose_wins_over_rationale_in_the_comment():
+    """What a token is FOR is what a reader of the stylesheet needs first."""
+    token = SimpleNamespace(
+        name="--fs-x", value_by_mode={"base": "1px"},
+        group_name=None, purpose="hairline borders", rationale="because thin",
+    )
+    css = render_stylesheet([token])
+    assert "hairline borders" in css
+    assert "because thin" not in css