docs: add BinaryIndexDecoder node design

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/plans/2026-04-04-binary-index-decoder-design.md

@@ -0,0 +1,67 @@
+# BinaryIndexDecoder Node — Design
+
+## Summary
+
+A standalone ComfyUI utility node that converts an integer index into 3 boolean
+outputs using binary (bit-field) encoding. Intended for use with loop counters to
+gate multiple processing branches simultaneously.
+
+## Node Spec
+
+| Field | Value |
+|---|---|
+| Class name | `BinaryIndexDecoder` |
+| Display name | `Binary Index Decoder` |
+| Category | `JSON Manager/utils` |
+| Function | `decode` |
+
+### Inputs
+
+| Name | Type | Default | Range |
+|---|---|---|---|
+| `index` | INT | 0 | 0–7 |
+
+### Outputs
+
+| Name | Type |
+|---|---|
+| `flag_0` | BOOLEAN |
+| `flag_1` | BOOLEAN |
+| `flag_2` | BOOLEAN |
+
+### Logic
+
+```
+flag_0 = bool((index >> 0) & 1)
+flag_1 = bool((index >> 1) & 1)
+flag_2 = bool((index >> 2) & 1)
+```
+
+### Truth table
+
+| index | flag_0 | flag_1 | flag_2 |
+|---|---|---|---|
+| 0 | F | F | F |
+| 1 | T | F | F |
+| 2 | F | T | F |
+| 3 | T | T | F |
+| 4 | F | F | T |
+| 5 | T | F | T |
+| 6 | F | T | T |
+| 7 | T | T | T |
+
+## Implementation Notes
+
+- Lives in `project_loader.py` alongside other project nodes
+- Added to `PROJECT_NODE_CLASS_MAPPINGS` and `PROJECT_NODE_DISPLAY_NAME_MAPPINGS`
+- No JavaScript extension needed (no source sync, no dynamic widgets)
+- No NiceGUI UI changes needed
+- `IS_CHANGED` not needed (output is deterministic from input)
+
+## Testing
+
+9 tests in `tests/test_project_loader.py::TestBinaryIndexDecoder`:
+- Input types include `index` as INT
+- All 8 index values (0–7) produce correct boolean tuple
+- Out-of-range index (e.g. 8) clamps to 0–7 or wraps gracefully
+- `NodeMappings` test updated: 5 nodes, mappings length == 5