ci: add TypeScript feature matrix radar

[andrewtdiz]

Summary

• add a test-features probe matrix for TypeScript and modern JavaScript compatibility checks
• add scripts/gen_feature_matrix.py to compare Node 26 stripped-TypeScript output against Perry-compiled output and generate markdown/JSON artifacts
• wire an advisory scheduled/manual/PR workflow that checks the committed matrix for drift

Fixes #801.

Matrix

• 22 probes across 14 categories
• Current baseline: 19 pass / 3 diff
• Recorded diffs: private static field state, Proxy construct trap mutation, typed-array value reads

Validation

• env CARGO_BUILD_JOBS=1 RUSTC_WRAPPER= CARGO_PROFILE_RELEASE_LTO=false CARGO_PROFILE_RELEASE_CODEGEN_UNITS=1 cargo build --release -p perry-runtime -p perry
• python3 scripts/gen_feature_matrix.py --check --node-bin /root/.npm/_npx/bac97da9607b7ef2/node_modules/node/bin/node --perry-bin target/release/perry --report .feature-matrix/feature_matrix.json --generated-output .feature-matrix/feature_matrix.md
• python3 -m py_compile scripts/gen_feature_matrix.py
• python3 -c "import pathlib, yaml; yaml.safe_load(pathlib.Path(".github/workflows/feature-matrix.yml").read_text())"
• git diff --check origin/main..HEAD
• git diff --check
• ./scripts/check_file_size.sh

Note: cargo emits existing unused/dead-code warnings in the current tree. The workflow build is scoped to the Perry compiler/runtime targets used by these probes.