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/filters/cfitz/Cfitz.md

@@ -8,7 +8,6 @@
 | **Outputs**      | Single series (Cfitz)                       |
 | **Output range** | Oscillates around zero           |
 | **Warmup**       | `2` bars                          |
-| **Signature**    | [cfitz_signature](cfitz_signature.md) |
 
 ### TL;DR
 

lib/filters/cfitz/cfitz_signature.md

@@ -1,3 +0,0 @@
-# CFITZ Signature
-
-![CFITZ Signature](cfitz_signature.svg)