feat(weld): phase alignment for layered bass, release 0.1.0

Weld hears the other bass layer through the sidechain and fits polarity
plus up to four RBJ allpass sections to the measured cross-spectrum, so
the layers sum instead of cancelling. No latency; the layer on its own
keeps its sound.

- Worker thread: Welch cross-spectrum and coherence, Nelder-Mead fit with
  1/f weighting, a per-band loss penalty and model selection by share of
  the achievable gain; GCC-PHAT lag detection flags a late layer.
- Audio thread: allocation-free chain with a 40 ms warm-up and 40 ms
  crossfade per new fit; mono blocks go to the worker through a lock-free
  queue.
- Follow / Learn / Hold; the fit is persisted and restored without a
  crossfade.
- Editor: per-band loss and recovery against a perfect alignment, phase
  before and after, recovered dB, time offset and status.
- Tests for flat magnitude, crossfade, fit targets, Follow hysteresis,
  Learn, silence, realtime allocation and the persisted fit in the host.

docs/development.md

@@ -32,7 +32,7 @@ NIH-plug Git dependency stays out of the root workspace. Host identities and
 saved state are separate for every product.
 
 The engines run allocation-free on the audio thread. Analysis that is too heavy
-for the audio thread (Rivet's generation jobs, and Weld's planned phase fit) runs on a worker
+for the audio thread (Rivet's generation jobs, Weld's cross-spectrum and fit) runs on a worker
 thread. Data goes to the editor through lock-free queues.
 
 ## Checks