Add tplot Greek-letter crib sheet

[huangzesen]

Summary

• Add a self-contained tplot crib demonstrating Greek-letter IDL text escapes in component labels and axis titles.
• Include a compact reference table for escapes already used by SPEDAS labels (!4h!X, !4u!X, !9Q!X, !9F!X, !4l!X, superscript/subscript, and line breaks).
• Note that exact glyph appearance can depend on the active IDL graphics device/font/runtime.

Validation

• git diff --check
• Static checks for header/procedure/synthetic store_data, labels, labflag, expected escapes, stop prompts, and final end
• Zhipu/GLM review: no blockers; requested portability caveat applied
• IDL runtime was not available on my PATH, so no local IDL smoke run was performed

Closes #17.