Avoid allocation when detecting content encoding

[ChristianPavilonis]

Summary

• Avoid allocating a lowercase String when detecting response compression from Content-Encoding.
• Preserve existing ASCII case-insensitive matching for gzip, deflate, and br.

Changes

File	Change
crates/trusted-server-core/src/streaming_processor.rs	Replaced to_lowercase() matching in Compression::from_content_encoding with non-allocating eq_ignore_ascii_case checks.

Closes

Closes #434

Test plan

• cargo test --workspace
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo fmt --all -- --check
• JS tests: cd crates/js/lib && npx vitest run
• JS format: cd crates/js/lib && npm run format
• Docs format: cd docs && npm run format
• WASM build: cargo build --package trusted-server-adapter-fastly --release --target wasm32-wasip1
• Manual testing via fastly compute serve
• Other: cargo test -p trusted-server-core streaming_processor::tests::test_compression_detection; cargo test -p trusted-server-core

Checklist

• Changes follow CLAUDE.md conventions
• No unwrap() in production code — use expect("should ...")
• Uses tracing macros (not println!)
• New code has tests
• No secrets or credentials committed