docs: replace dummy docs with GEMASTIK 2026 Warmup writeups (ch1-14)

- Deleted all previous dummy docs (defcon-quals-2024, infra/cloudflare-525, debugging/websocket-timeout, template, docs/*, notes/*, research/*)
- Added 14 Gemastik warmup writeups covering encoding, stego, web, pwn, crypto
- ch13: RSA small factors; ch14: RSA special integers via paper-search (GCD with paper appendix primes)
- All flags recovered and documented with solution methods

content/writeups/gemastik-2026-warmup/ch12-layers.md

@@ -0,0 +1,34 @@
+---
+title: "Layers — Hex to Base64 Chain"
+event: "GEMASTIK 2026 Warmup"
+challenge: "Layers"
+category: "crypto"
+points: 500
+difficulty: "easy"
+flag: "GEMASTIK19{h3x_lQlu_b4s3_ch41n}"
+---
+
+# Layers (500 pts) — Encoding chain
+
+**File:** `chall.txt` → `5230564e51564e55535573784f58746f4d33686662464673645639694e484d7a58324e6f4e44467566513d3d`
+
+Dua lapis encoding berturut-turut: **Hex → bytes → Base64 → flag**:
+
+```python
+import base64
+
+s = "5230564e51564e55535573784f58746f4d33686662464673645639694e484d7a58324e6f4e44467566513d3d"
+
+# Layer 1: hex decode
+b = bytes.fromhex(s)
+# b = b'R0VNQVNUSUsxOXtoM3hfbFFsdV9iNHMzX2NoNDFufQ=='
+
+# Layer 2: base64 decode
+flag = base64.b64decode(b).decode()
+print(flag)
+# GEMASTIK19{h3x_lQlu_b4s3_ch41n}
+```
+
+## Flag
+
+`GEMASTIK19{h3x_lQlu_b4s3_ch41n}`