Fixes:
- Corrected test_color_device_rgb_clamped expected value from "#ff8080" to "#ff0080"
(G value -0.5 should clamp to 0.0, not 0.5)
- Fixed lifetime annotation in readability.rs (Cow<str> -> Cow<'_, str>)
- Fixed unused_must_use warning in page_class.rs test
Verification (notes/pdftract-tuky.md):
- All 8 children of Phase 3.1 coordinator are closed
- q/Q 64-level depth limit verified (test_64_nested_q_calls_succeed)
- Td chain accumulation verified (test_td_chain)
- Tm/Td ordering correct per ISO 72-bit spec
- /Rotate normalization implemented in child pdftract-1jlpy
- All 6 color operators tracked (72 graphics_state tests pass)
Closes: pdftract-tuky
