Add extension attribute schema tests and CI feature matrix

[gvonness-apolitical]

Summary

• Add cdx-core/tests/extension_schema.rs with table-driven tests verifying field names and types for all 15 ExtensionMark convenience constructors (citation, entity, glossary, index, equation_ref, algorithm_ref, theorem_ref, highlight, etc.)
• Add backward-compatibility deserialization tests for citation marks (both "refs" array and legacy "ref" string formats)
• Add Citation struct schema tests verifying serialization emits "refs" (not "ref") and deserialization accepts both
• Add citation mark archive roundtrip test (single-ref and multi-ref marks survive save/reopen via CdxWriter/CdxReader)
• Add CI feature matrix job testing 5 feature combinations: no-features, zstd, signatures, zstd+signatures, signatures+encryption

Test plan

• cargo test --workspace --all-features passes (1052 tests, 0 failures)
• cargo clippy --workspace --all-features -- -D warnings clean
• cargo build -p cdx-core --no-default-features builds successfully
• CI feature matrix job runs all 5 combinations without failure

[codecov]

Codecov Report

❌ Patch coverage is 94.26230% with 14 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
cdx-core/tests/extension_schema.rs	95.14%	10 Missing ⚠️
cdx-core/tests/integration.rs	89.47%	4 Missing ⚠️

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