doc(core): document remaining public API + enable crate-wide missing_docs (#226)

[kalwalt]

Slice 3b — the finisher for #226. Documents the last of the undocumented public surface and turns on the crate-wide #![warn(missing_docs)] gate.

What's documented

• types.rs (116) — the ARToolKit core types: ARHandle, ARParamLT/ARParamLTf, ARLabelInfo, ARPattHandle, the ARPixelFormat / ARMatrixCodeType / labeling enums, timestamps, the mode/threshold constants, and the ARMarkerInfoCutoffPhase variants.
• ar/* — math (ARMat/ARMatf/ARVec + PCA/QR/eigen helpers), and the marker/pattern/matrix/pose/labeling/image_proc/bch constants, enums, and functions.
• arlog.rs — the log-level variants + the arlog_{d,i,w,e} macros.

The gate

Adds #![warn(missing_docs)] at the crate root. The crate now reports zero missing docs on both default features and --all-features; CI's clippy -D warnings escalates the lint, so any new undocumented pub item fails the build from here on. The bindgen-generated FFI bindings keep their local #![allow(missing_docs)] (kpm::kpm_ffi, from #231).

Verification

• cargo rustdoc -W missing_docs: 0 (default) and 0 (--all-features)
• cargo clippy --workspace --all-targets --all-features -D warnings: clean (run locally with LIBCLANG_PATH set to an x64 libclang — the exact CI command)
• cargo fmt --check clean; cargo test --lib: 432 passed

Documentation only — no code or signatures changed.

#226 status

With #231 (KPM rustdoc), #232 (JS/TS guide), #233 (AR2/ICP), and this PR, the API-reference + integration-guide scope of #226 is complete → #226 can be closed on merge.

Out of scope (tracked separately if desired): the ~43 pre-existing broken intra-doc links (e.g. arlog_rel, _verbose helpers) — those are cargo doc warnings, not part of the clippy gate.

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[codecov]

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