chore: scaffold minimal NIH-plug template

Based on musiikki/codename-206 patterns. Provides a clean, minimal
skeleton for new nih-plug audio plugins:

- Cargo.toml with pinned NIH-plug deps (same rev as codename-206)
- src/lib.rs — skeleton plugin with Params, Plugin impl, CLAP/VST3
- xtask/ — bundler xtask (cargo xtask bundle)
- bundler.toml — placeholder for bundle name
- .cargo/config.toml — xtask alias
- .gitignore — /target, /nih-plug, /_template
- LICENSE — Apache-2.0
- README.md — build and deploy instructions

xtask/src/main.rs

@@ -0,0 +1,5 @@
+// SPDX-License-Identifier: Apache-2.0
+
+fn main() -> nih_plug_xtask::Result<()> {
+    nih_plug_xtask::main()
+}