Complete Epic 114 safe CPU dispatch

[rsasaki0109]

Summary

• centralize deterministic CPU scheduling thresholds, row tiles, and worker bounds for vision kernels
• share packed one/three-channel u8 fast-path selection while retaining generic channel/stride fallbacks
• apply the policy to resize, preprocessing, Gaussian, Sobel, morphology, and Canny execution
• close Epic 114 with deterministic dispatch tests and a worker/scratch ownership receipt

Impact

Small images stay scalar, large work uses bounded existing pools, and packed specializations remain conditional on exact channel and stride contracts. Public APIs, numerical contracts, CPU/GPU transfer behavior, and the SpatialRust 1.1.0 version are unchanged.

Validation

• cargo test -p spatialrust-vision --no-default-features --features full (139 unit + 13 property tests)
• cargo clippy --workspace --all-targets -- -D warnings (Rust 1.97.0)
• cargo clippy --workspace --all-features --lib -- -D warnings (Rust 1.97.0)
• RUSTFLAGS="-D warnings" cargo test --workspace --no-run
• cargo fmt --all --check
• git diff --check