PowerNoise: prepare to write some docs

doc/4-instrument/powernoise.md

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+# PowerNoise instrument editor
+
+the PowerNoise instrument editor consists of two tabs.
+
+## LFSR-based synthesis
+
+this will be finished later... it's so late and I need to sleep now
+
+## Macros
+
+- **Volume**: volume sequence.
+- **Arpeggio**: pitch sequence.
+- **Duty/Noise**: duty cycle and noise mode.
+  - pulse duty cycles:
+    - `0`: 12.5%
+    - `1`: 25%
+    - `2`: 50%
+    - `3`: 75%
+  - noise modes:
+    - `0`: long noise
+    - `1`: short noise
+- **Pitch**: fine pitch.
+- **Phase Reset**: trigger restart of waveform.
+
+## PowerNoise tab
+
+this tab allows you to change the base octave - important when you have set a longer LFSR pattern.

doc/7-systems/powernoise.md

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+# PowerNoise
+
+a fantasy sound chip created by StinkerB06 and jvsTSX for the Hexheld fantasy video game console.
+
+its design employs linear-feedback shift registers (LFSR) for sound generation. this technology is used in many random number generators to produce noise, but it is also capable of producing other tones.
+
+it has three noise channels and one "slope" channel capable of generating a variety of saw waves.
+
+it outputs stereo sound with 4-bit volume control per channel and 3-bit master volume control.
+
+refer to its instrument type's documentation for details on sound synthesis.
+
+## effect commands
+
+- `20xx`: **load LFSR value (low byte).**
+  - on the slope channel, this sets its accumulator (from `00` to `7F`).
+- `21xx`: **load LFSR value (high byte).**
+- `22xx`: **write to I/O port A.**
+- `23xx`: **write to I/O port B.**
+
+## info
+
+this chip uses the [PowerNoise](../4-instrument/powernoise.md) instrument editor.