feat(kpm): M7 — Hough voting & feature matching in pure Rust

[kalwalt]

Goal

Port the descriptor matching pipeline from
WebARKitLib/lib/SRC/KPM/FreakMatcher/matchers/
into pure Rust under crates/core/src/kpm/freak/.

This milestone produces three new modules:

Module	Content
freak/hough.rs	Hough similarity voting — geometric consistency filter
freak/clustering.rs	KMedoids + Binary Hierarchical Clustering — vocabulary tree
freak/matcher.rs	FeatureStore + FeatureMatcher — descriptor search

After this milestone, the full descriptor matching pipeline exists in
pure Rust and M8 (DoG detector + FREAK descriptor) can begin.

Sub-issues

• feat(kpm): M7-1 — port hough_similarity_voting.h to freak/hough.rs #109 — M7-1: hough_similarity_voting.h → freak/hough.rs
• feat(kpm): M7-2 — port kmedoids.h + binary_hierarchical_clustering.h to freak/clustering.rs #110 — M7-2: kmedoids.h + binary_hierarchical_clustering.h → freak/clustering.rs
• feat(kpm): M7-3 — port feature_store.h + feature_matcher.h + feature_matcher-inline.h to freak/matcher.rs #111 — M7-3: feature_store.h + feature_matcher.h/.inline.h → freak/matcher.rs

File inventory

C++ file	Lines	Prompt
matchers/hough_similarity_voting.h	493	M7-1
matchers/kmedoids.h	223	M7-2
matchers/binary_hierarchical_clustering.h	445	M7-2
matchers/feature_store.h	138	M7-3
matchers/feature_matcher.h	158	M7-3
matchers/feature_matcher-inline.h	450	M7-3

Total: ~1906 lines of C++.

Rust module layout after M7

crates/core/src/kpm/freak/
  mod.rs
  math.rs         (M6)
  homography.rs   (M6)
  hough.rs        ← M7-1
  clustering.rs   ← M7-2
  matcher.rs      ← M7-3
  pyramid.rs      (M8)
  detector.rs     (M8)
  descriptor.rs   (M8)

Dependency order

M7-1 (hough.rs)  — no M7 deps, start immediately after M6
     ↓
M7-2 (clustering.rs)  — no M7 deps, can run in parallel with M7-1
     ↓
M7-3 (matcher.rs)  — depends on M7-2 (BHC index inside FeatureMatcher)

M7-1 and M7-2 can be worked on in parallel if two contributors are available.

Acceptance criteria

cargo test -p webarkitlib-rs -- kpm::freak::hough \
                                kpm::freak::clustering \
                                kpm::freak::matcher
cargo clippy -p webarkitlib-rs -- -D warnings

All tests pass. Zero clippy warnings.

Dual-mode validation gate (run before closing M7-3):

cargo test -p webarkitlib-rs --features dual-mode \
  -- test_feature_matcher_output_matches_cpp

Match count within ±2 of C++ baseline on the pinball-demo fixture.

Branch

feat/freak-matching