Add xToys pattern import via URL paste

- XToysPattern.cs: fetch pattern from xtoys.app API (one call), parse
  script-v3 JSON, resolve slider defaults, evaluate sine/straight steps
  to WaveFrames at 100ms resolution
- Frequency mapping: 0%=1000ms deep, 100%=10ms buzzy
- MainForm: Pattern button + input dialog, continuously enqueues pattern
  frames as a loop, Stop All cancels
- Fix: handle JSON values that are numbers vs strings (end/start fields)
- NOTES.md: xToys slider parameter intel + frequency mapping docs

NOTES.md

@@ -129,3 +129,28 @@ web game / userscript  ──WS──>  Substation  ──BLE──>  Coyote 3.0
 ```
 
 Build: `dotnet run -c Release`
+
+## xToys pattern import
+
+Paste a URL like `https://xtoys.app/patterns/-OuhuHOuY1AlPPoCJlzc` into the Pattern dialog.
+The app fetches the pattern once from `https://xtoys.app/api/getPatternv2`, evaluates it
+locally, and plays it as a continuous loop. No ongoing API calls.
+
+### xToys script-v3 slider parameters
+
+From pattern analysis:
+
+- **A CH** (0.5–10): Divides the pattern between channels. If A=1 and B=2, then 2/3 of
+  the pattern goes to channel B. Controls the "hold" duration on channel A.
+- **B CH** (0.5–10): Same division for channel B. Controls the "pause" duration.
+- **Ramp** (0.5–5): Applies a smoothness filter to transitions. Higher = slower ramps.
+- **Min Freq** (0–100): Low cutoff on frequency values. No idea why.
+- **Min Level** (0–100): Low cutoff on intensity values. Floor that's held during "pause".
+
+### Frequency mapping
+
+xToys frequency is 0–100 (percentage). Mapped to e-stim period:
+- 0% = 1000ms (1Hz, deep thump)
+- 100% = 10ms (100Hz, buzzy)
+
+Formula: `period_ms = 1000 - 9.9 * freq_pct`