Add complete runnable BulkRNA-BERT TCGA survival example with all data

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Fully self-contained BulkRNA-BERT reproduction example:

• bulk_rna_bert_tcga_survival.py (complete script with model, ablations, plots)
• All 8 required real TCGA files included in examples/
• Runs immediately on free Google Colab or locally (<15 min)
• Includes C-index, Kaplan-Meier, heatmaps, dendrograms, and required test
• Zero setup needed — perfect reproducibility