fix(mat): make serde non-optional — unblocks cargo test --workspace --no-default-features

Making wifi-densepose-mat optional in the CLI (for the aarch64/ort decouple)
exposed a latent feature bug: mat's `api` module compiles unconditionally and
uses serde, but `serde` was an optional dep enabled only via the `api`/`serde`
features. Previously the CLI's *unconditional* mat dependency enabled those
features transitively, so `--workspace --no-default-features` still got serde;
once mat became optional+gated, the workspace build lost it →
`error[E0432]: unresolved import serde` across mat's api/* (CI red).

mat already pulls serde_json + axum unconditionally, so making `serde`
non-optional has no real cost and restores the workspace build. Does NOT affect
the aarch64 CLI build (mat isn't built there at all): verified
`cargo tree -p wifi-densepose-cli --no-default-features` still shows 0
ort/openssl deps, and `cargo test --workspace --no-default-features` compiles
clean.

Co-Authored-By: claude-flow <ruv@ruv.net>

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

@@ -15,12 +15,12 @@ readme = "README.md"
 default = ["std", "api", "ruvector"]
 ruvector = ["dep:ruvector-solver", "dep:ruvector-temporal-tensor"]
 std = []
-api = ["dep:serde", "chrono/serde", "geo/use-serde"]
+api = ["chrono/serde", "geo/use-serde"]
 portable = ["low-power"]
 low-power = []
 distributed = ["tokio/sync"]
 drone = ["distributed"]
-serde = ["dep:serde", "chrono/serde", "geo/use-serde"]
+serde = ["chrono/serde", "geo/use-serde"]
 
 [dependencies]
 # Workspace dependencies
@@ -43,7 +43,7 @@ thiserror = "2.0"
 anyhow = "1.0"
 
 # Serialization
-serde = { version = "1.0", features = ["derive"], optional = true }
+serde = { version = "1.0", features = ["derive"] }
 serde_json = "1.0"
 
 # Time handling