fix(adr-185): hoist AETHER compute surface into a std-only leaf crate

ADR-185 §13 flagged the `[aether]` wheel as linking the whole
`wifi-densepose-sensing-server` Axum/tokio/worldgraph/ruvector tree via the
non-optional deps of that crate. Hoist the pure-compute AETHER stack out so the
binding depends only on pure Rust.

New crate `wifi-densepose-aether` (std-only, ZERO external deps) holds the four
self-contained modules the AETHER surface needs — `embedding`, `graph_transformer`,
`sona`, `sparse_inference` (verified: no serde/tokio/axum/crate:: refs outside the
set; the four form a closed dependency graph). `wifi-densepose-sensing-server`
now `pub use`s them from the leaf crate, so its own code (`crate::embedding`, …)
and public API (`wifi_densepose_sensing_server::embedding`, …) are unchanged —
`main.rs`/`trainer.rs` untouched. The Python binding + parity test + `[aether]`
feature repoint at the leaf crate; the sensing-server dep is dropped from python.

MEASURED wheel size (maturin build --release --features aether --strip; each
wheel install-verified to actually contain `AetherConfig`):
  BEFORE (aether -> sensing-server): 369,782 B (~361 KB)
  AFTER  (aether -> leaf crate):     319,719 B (~312 KB)

HONEST CORRECTION to the §9/§13 premise: the pre-hoist wheel was NOT over the
ADR-117 §5.4 5 MB budget — it was ~361 KB, ~14x under. Linker dead-code
elimination (--gc-sections on the pyo3 cdylib) already strips the unreached
server tree because the binding only reaches pure-compute symbols. So this is
"blows the budget" CLAIMED, not MEASURED. The hoist is still worthwhile and
lands the real, measured wins:
  - `[aether]` build time 71s -> 12s (no server tree to compile),
  - python/Cargo.lock -1238 lines (server tree no longer resolved),
  - ~50 KB smaller wheel,
  - honest, minimal declared dependency surface + removes the latent risk that a
    future change makes some server code reachable and *then* bloats the wheel.

Verified this session (real counts):
  - cargo test --features aether --test aether_parity: 2 passed / 0 failed
  - pytest tests/test_aether.py (maturin develop --features aether): 9 passed
  - cargo test -p wifi-densepose-sensing-server --no-default-features: 217 bin +
    388 lib, 0 failed (no regression)
  - cargo test -p wifi-densepose-aether: 96 passed (the relocated unit tests)

The §9/§13 wheel-size narrative in ADR-185 (owned by the ADR author) should be
corrected to reflect the measured reality — flagged to the team lead.

v2/Cargo.lock

@@ -10851,6 +10851,10 @@ version = "1.2.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "72069c3113ab32ab29e5584db3c6ec55d416895e60715417b5b883a357c3e471"
 
+[[package]]
+name = "wifi-densepose-aether"
+version = "0.3.0"
+
 [[package]]
 name = "wifi-densepose-bfld"
 version = "0.3.1"
@@ -11149,6 +11153,7 @@ dependencies = [
  "tracing",
  "tracing-subscriber",
  "ureq 2.12.1",
+ "wifi-densepose-aether",
  "wifi-densepose-bfld",
  "wifi-densepose-engine",
  "wifi-densepose-geo",
@@ -11220,7 +11225,6 @@ dependencies = [
  "tracing",
  "tracing-subscriber",
  "walkdir",
- "wifi-densepose-nn",
  "wifi-densepose-signal",
 ]