feat(weld): add sidechain test build

Weld will phase-align a bass layer to another layer it hears through the
sidechain. This first build only checks that FL Studio delivers the
sidechain to a CLAP aux port: it passes the main layer through unchanged,
meters the sidechain and offers Layer/Sum/Sum dry listening. No latency.

docs/weld/CHANGELOG.md

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+# Weld changelog
+
+## Unreleased
+
+- Sidechain test build: stereo or mono main input with a matching sidechain
+  input, a sidechain level readout, Listen (Layer, Sum, Sum dry), Output and
+  Bypass. No phase alignment yet.

docs/weld/README.md

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+# Weld
+
+Weld aligns the phase of one bass layer to another layer so the two sum
+instead of cancelling in the mids.
+
+> **Status: sidechain test build.** This build only checks that FL Studio
+> delivers a sidechain signal to Weld. It passes the main layer through
+> unchanged and shows the sidechain level. Phase alignment comes in the next
+> build.
+
+## Set up in FL Studio
+
+1. Load Weld on the mixer track of one layer (the main layer).
+2. On the mixer track of the other layer, right-click the route arrow to the
+   main layer's track and choose **Sidechain to this track**.
+3. In Weld's plugin wrapper, open the settings (gear icon) and, under the
+   processing or input routing options, set Weld's sidechain input to that
+   track.
+4. Play both layers. The large readout must show `sidechain: −NN dBFS`, and the
+   SIDECHAIN meter must move.
+
+## Controls
+
+- **Listen**:
+  - **Layer** outputs the main layer; use it in the mix.
+  - **Sum** outputs the main layer plus the sidechain layer. Mute the other
+    layer's own send to the master while you listen to Sum.
+  - **Sum dry** outputs the unprocessed sum, for comparison.
+- **Output**: output trim.
+- **Bypass**.
+
+Weld adds no latency.