(chore): add datasets stub and ignore raw data.

.gitignore

@@ -44,3 +44,8 @@ wea-*.txt
 sta-*.txt
 scripts/generate_saliency.py
 scripts/compare_samples.py
+# --- Data (keep folder, ignore files) ---
+datasets/**
+!datasets/.gitkeep
+!datasets/.README.md
+# ---------------------------------------

datasets/README.md

@@ -0,0 +1,5 @@
+Place Raman .txt spectra here (not tracked).
+Expected Formats:
+- one column: intensity only
+- two columns: x, intensity (any whitespace delimiter)
+Comments (# ...) are allowed.