refactor(tests): split remaining inline test mods into sibling tests.rs files

[uqio]

Summary

Mechanical layout refactor: move 4 large inline mod tests { ... } blocks out of their parent source files into sibling tests.rs files. Pure file reorganization — zero behavior change, zero test logic change, test count unchanged at 499.

Continues the convention established by PR #21 (which split the per-arch SIMD backend tests). The remaining 4 files were the largest still carrying inline test modules.

Changes

Parent file	Tests moved to	Lines moved
src/frame.rs	src/frame/tests.rs	1,572
src/raw/bayer16.rs	src/raw/bayer16/tests.rs	474
src/raw/bayer.rs	src/raw/bayer/tests.rs	393
src/raw/types.rs	src/raw/types/tests.rs	251

Each parent file now ends with the standard two-line declaration (e.g. #[cfg(all(test, feature = "std"))] mod tests;), with the body verbatim in the sibling file (dedented one level, use super::*; retained). The original #[cfg] attribute style is preserved per-file (src/raw/types.rs uses bare #[cfg(test)]; the others use #[cfg(all(test, feature = "std"))]).

Why now

• src/frame.rs was 6,439 lines; the inline test block accounted for 24% of the file. Reviews of frame-layout changes had to scroll past a thousand-plus lines of test code.
• Per-arch SIMD backends (PR refactor(tests): split inline test mods into sibling tests.rs files + 2 PR #20 review fixes #21) and src/sinker/mixed/mod.rs already use the sibling pattern — these 4 files were the last holdouts.
• Smaller parent files keep CI diff-stat noise lower for future PRs that touch the implementation but not the tests.

Test plan

• cargo test --lib: 499 pass, 0 fail (unchanged from before the split — pure layout refactor)
• cargo check --tests --lib clean across host (aarch64-darwin), x86_64-unknown-freebsd, wasm32-unknown-unknown
• RUSTFLAGS="-Dwarnings" cargo clippy --lib --tests clean

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