Let decks define note macros in notes-preamble.tex

Notes that use a deck's own macros failed to typeset, since mknotes
only pulled in fonts.tex. A deck now defines them in notes-preamble.tex
(starting with \input{fonts.tex}), which mknotes prefers when present.
The Makefile keeps the new fragment out of the document list.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

slides_template/mknotes

@@ -158,6 +158,10 @@ HMARGIN = 0.06
 
 #: Font setup shared with the slides. Used when it exists, so that the notes
 #: match the deck without anyone having to remember a flag.
+# notes-preamble.tex wins when present: a deck whose notes use its own macros
+# defines them there (\input{fonts.tex} included), keeping fonts.tex a pure
+# font fragment shared with the slides.
+DEFAULT_PREAMBLES = ("notes-preamble.tex", "fonts.tex")
 DEFAULT_PREAMBLE = "fonts.tex"
 
 # The page is laid out by hand rather than by geometry: the header band runs
@@ -360,10 +364,10 @@ def main():
     # A preamble named on the command line has to be there; the default one is
     # a convenience and is skipped when the document does not use it.
     preamble = args.preamble
-    if preamble and not os.path.exists(preamble):
-        if preamble != DEFAULT_PREAMBLE:
-            raise SystemExit(f"mknotes: {preamble} not found")
-        preamble = None
+    if preamble == DEFAULT_PREAMBLE:
+        preamble = next((p for p in DEFAULT_PREAMBLES if os.path.exists(p)), None)
+    elif preamble and not os.path.exists(preamble):
+        raise SystemExit(f"mknotes: {preamble} not found")
 
     workdir = tempfile.mkdtemp(prefix="mknotes-")
     try: