refactor: move Decay/Edecay from trends_IIR to numerics; update filter signatures

- Move lib/trends_IIR/decay/ → lib/numerics/decay/
- Move lib/trends_IIR/edecay/ → lib/numerics/edecay/
- Update Category in Decay.md/Edecay.md from Trends (IIR) to Numerics
- Add DECAY/EDECAY entries to lib/numerics/_index.md and docs/indicators.md
- Update filter signature .md files and .svg assets
- Update trends_IIR signature docs (htit, mama, holt, etc.)
- All 163 tests passing, 0 warnings, 0 errors

lib/trends_IIR/htit/Htit.md

@@ -8,6 +8,8 @@
 | **Outputs**      | Single series (HTIT)                       |
 | **Output range** | Tracks input                     |
 | **Warmup**       | `12` bars                          |
+| **Signature**    | [htit_signature](htit_signature.md) |
+
 
 ### TL;DR
 

lib/trends_IIR/htit/htit_signature.md

@@ -0,0 +1,3 @@
+# HOLT Signature
+
+![HTIT Signature](htit_signature.svg)