Accept provider icon namespaces

[konojunya]

Summary

• accept the specification-defined provider:icon-name form while preserving existing theme-local IDs
• reject malformed, uppercase, empty, and multiply namespaced provider icon IDs with STK3013
• keep namespaced IDs unchanged in normalized IR
• pin the merged specification revision containing the new conformance fixture

Verification

• cargo test --workspace --locked
• STACK_SPECIFICATION_DIR=../specification cargo test --features conformance --test conformance --locked
• cargo clippy --workspace --all-targets --all-features --locked -- -D warnings
• RUSTDOCFLAGS="-D warnings" cargo doc --workspace --no-deps --locked
• cargo llvm-cov --lib --all-features --workspace --locked --fail-under-lines 95 --fail-under-functions 95 --fail-under-regions 95 (97.68% regions, 98.78% functions, 98.47% lines)
• cargo +1.85.0 test --workspace --locked
• cargo +1.85.0 clippy --workspace --all-targets --all-features --locked -- -D warnings