Add conformance coverage matrix generator, report, and CI check - #542

Summary

Adds the CEL Conformance Coverage Matrix (doc/conformance.md), generator tooling (tools/), Bazel targets, and a Stage 1 GitHub Actions consistency & unit test workflow (.github/workflows/conformance_matrix.yml):

• Conformance Coverage Report (doc/conformance.md): Section-by-section coverage matrix across official CEL runtimes (Go, C++ (Modern), C++ (Legacy), and Java) linked directly to .textproto suites under tests/simple/testdata/, with curated non-conformance notes and callouts.
• Matrix Generator & Validation Gates (tools/):
  • generate_conformance_matrix.py + generate_conformance_matrix_test.py (unittest.TestCase compatible)
  • conformance_manifest.yaml (pinned runtime release configurations) & conformance_notes.yaml (curated skip/failure explanations and callout blocks)
  • Enforces skip validation, corpus union coverage validation, strict manifest checks, note coverage, and baseline regression checks.
• Bazel Support:
  • Adds py_proto_library targets under proto/cel/expr/conformance/{proto2,proto3,test}/BUILD.bazel, //tests/simple:simple_testdata, //doc:conformance.md, //tools:generate_conformance_matrix, //tools:generate_conformance_matrix_test, and a dev_dependency = True pyyaml archive in MODULE.bazel (isolated from downstream BCR consumers).
• GitHub Actions CI (.github/workflows/conformance_matrix.yml):
  • Runs bazelisk test //tools:generate_conformance_matrix_test and bazelisk run //tools:generate_conformance_matrix -- ... --strict_manifest --baseline_markdown doc/conformance.md --check_consistency doc/conformance.md on PRs and pushes affecting conformance testdata, protos, tools/, or doc/conformance.md.