fix: mass chart, GIF sync, descriptor card colors, SVG cleanup

ecosystem/result.py
  mass_history was never written to to_summary_dict() -- only
  species_mass_history was serialized. build_index read an empty list
  and produced no SVG path, showing "total mass conserved across run"
  even when mass dropped from 6789 to 303 au in a signal run.
  Fixed: mass_history added to the measures dict in to_summary_dict().

scripts/build_index.py
  Added backward-compat fallback: if mass_history is absent (older runs
  without the fix), reconstruct it by summing species_mass_history rows.

viz/templates/ecosystem.html
  GIF sync: on tab switch, both mass and signal GIFs now restart from
  frame 0 via src-clear trick (img.src = ''; img.src = src) before the
  opacity swap. Switching mass↔signal always shows the same simulation
  moment on both sides.

viz/templates/index.html
  receptor_sensitivity descriptor card changed from purple to teal to
  match emission_activity and signal_retention -- all three are signal-only
  descriptors and should share the same visual treatment.

docs/signal-field.svg
  Full redraw: tighter font sizes (8-9.5px), proper label/mono/dim
  hierarchy, all text fits within containers, cross-arrows labeled
  concisely, footer note on conservation. No text overflow anywhere.

Author: rkv0id

docs/signal-field.svg

@@ -772,6 +772,11 @@ def _render_ecosystem_run(run_dir: Path, publish: bool = False) -> str:
     min_dist = spawn.get("min_dist", "")
     measures = summary.get("measures", {})
     mass_history: list[float] = measures.get("mass_history", [])
+    # Backward compat: older runs lack mass_history; reconstruct from species sums.
+    if not mass_history:
+        smh: list[list[float]] = measures.get("species_mass_history", [])
+        if smh:
+            mass_history = [sum(s[i] for s in smh) for i in range(len(smh[0]))]
     snapshot_steps: list[int] = measures.get("snapshot_steps", [])
     initial_mass = measures.get("initial_mass", 0.0)
     final_mass = measures.get("final_mass", 0.0)

scripts/build_index.py

@@ -173,6 +173,7 @@ def to_summary_dict(self) -> dict[str, object]:
                 "min_mass": m.min_mass,
                 "mass_turnover": m.mass_turnover,
                 "snapshot_steps": m.snapshot_steps,
+                "mass_history": m.mass_history,
                 "species_mass_history": m.species_mass_history,
                 "species_territory_history": m.species_territory_history,
                 "interaction_coefficients": m.interaction_coefficients,