feat: headless PNG render, mLnL annotations, and per-frame sched offset

Commit the accumulated working-tree changes as one snapshot.

- Headless render: `--render OUT.png INPUT.wav` draws the spectrogram
  (full window, or `--pane` for the spectrogram pane only) to a PNG
  with no visible window. Options: `--annotations`/`--no-annotations`,
  `--annotation-opacity`, `--width`/`--height`.
- mLnL annotations: parse the optional `mLnL` RIFF chunk (schema v2)
  and render tx_frame/assertion/control overlays, a timeline lane, and
  a waveform-scope echo, with hover tooltips on the spectrogram,
  timeline, and scope.
- sched_offset_ms: parse the per-frame intent->air latency and surface
  it in the hover tooltips (boxes stay air-anchored upstream).
- Supporting: build wiring (rspektrum.make), shared types/headers,
  web-build and capture-script tweaks, and removal of the old
  synchrosqueezing LaTeX doc.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

rspektrum.make

@@ -127,6 +127,7 @@ GENERATED += $(OBJDIR)/ui.o
 GENERATED += $(OBJDIR)/platform_linux.o
 GENERATED += $(OBJDIR)/utils.o
 GENERATED += $(OBJDIR)/primitives.o
+GENERATED += $(OBJDIR)/mlnl.o
 OBJECTS += $(OBJDIR)/spectrogram.o
 OBJECTS += $(OBJDIR)/fft.o
 OBJECTS += $(OBJDIR)/stft.o
@@ -136,6 +137,7 @@ OBJECTS += $(OBJDIR)/ui.o
 OBJECTS += $(OBJDIR)/platform_linux.o
 OBJECTS += $(OBJDIR)/utils.o
 OBJECTS += $(OBJDIR)/primitives.o
+OBJECTS += $(OBJDIR)/mlnl.o
 
 # Rules
 # #############################################
@@ -235,6 +237,10 @@ $(OBJDIR)/primitives.o: src/primitives.c
 	@echo "$(notdir $<)"
 	$(SILENT) $(CC) $(ALL_CFLAGS) $(FORCE_INCLUDE) -o "$@" -MF "$(@:%.o=%.d)" -c "$<"
 
+$(OBJDIR)/mlnl.o: src/mlnl.c
+	@echo "$(notdir $<)"
+	$(SILENT) $(CC) $(ALL_CFLAGS) $(FORCE_INCLUDE) -o "$@" -MF "$(@:%.o=%.d)" -c "$<"
+
 -include $(OBJECTS:%.o=%.d)
 ifneq (,$(PCH))
   -include $(PCH_PLACEHOLDER).d