fix(adr-185): drop dead wifi-densepose-nn dep from train to slim [meridian] wheel

Root cause (investigated, same rigor as the MAT fix): `wifi-densepose-train`
declared `wifi-densepose-nn` as a dependency but NEVER imported it anywhere
in src/ or bin/ — a dead dependency. (The tch-backend model path uses the
`tch` crate directly, not wifi-densepose-nn; `grep -r wifi_densepose_nn
src/` returns nothing.) That dead dep pulled `ort` (ONNX Runtime) +
reqwest/hyper into every downstream consumer, including the ADR-185
`[meridian]` wheel, which binds only train's pure-compute geometry /
rapid_adapt / eval modules + signal::hardware_norm.

`cargo tree -i wifi-densepose-nn` confirmed train was the SOLE path pulling
nn into the wheel; after removal, the package is absent from the graph.

Fix: remove the dead dep line from train's Cargo.toml (no code change, no
feature-gating needed — nothing referenced it). Not a hoist: the coupling
turned out to be nonexistent, not deep.

Measured (maturin build --release --features meridian --strip):
  BEFORE: 1.8 MB   AFTER: 1.7 MB   (both already within the ADR-117 §5.4
  <=5 MB budget). Honest note: unlike MAT — which instantiates OnnxSession
  and thus bundles the ort native runtime (8.4 MB) — train never references
  ort, so the native lib was dead-stripped and the wheel was already under
  budget. The real win is removing ort/reqwest/hyper from train's dependency
  graph and compile for ALL consumers, not a dramatic wheel shrink.

Verified:
  cargo build -p wifi-densepose-train                       (default=[]) OK
  cargo tree -i wifi-densepose-nn  -> no longer in the graph
  cargo test --features meridian --test meridian_parity     2/2 pass
  maturin develop --features meridian + pytest test_meridian 13/13 pass

v2/crates/wifi-densepose-train/Cargo.toml

@@ -35,7 +35,13 @@ cuda = ["tch-backend"]
 [dependencies]
 # Internal crates
 wifi-densepose-signal = { version = "0.3.0", path = "../wifi-densepose-signal", default-features = false }
-wifi-densepose-nn = { version = "0.3.0", path = "../wifi-densepose-nn" }
+# NOTE: `wifi-densepose-nn` was declared here but never imported anywhere in
+# this crate's src/ or bin/ (the tch-backend model path uses `tch` directly,
+# not this crate). It was a dead dependency that pulled `ort` (ONNX Runtime) +
+# reqwest/hyper into every downstream consumer — including the ADR-185
+# `[meridian]` wheel. Removed to slim the dependency graph. Inference at
+# serving time is done via `wifi-densepose-nn` by the binaries that actually
+# load models, which depend on it directly.
 
 # Core
 thiserror.workspace = true