Add measurable test inventory, testing docs, and Makefile target

[aoxc]

Motivation

• Provide measurable, discoverable test inventory and tie the repository's TESTING.md policy to concrete artifacts to close the visibility gap between policy and evidence.
• Make it trivial for operators and reviewers to regenerate a workspace test snapshot so test-surface counts are repeatable and auditable.

Description

• Add scripts/testing/generate_test_inventory.py which scans source markers and emits artifacts/testing/test_inventory.json and docs/testing/TEST_MATRIX.md summarizing crate-scoped tests, integration markers, property macro occurrences, and fuzz targets.
• Add test-inventory Makefile target and surface it in the help output and PHONY list so inventory generation is a first-class operator action via make test-inventory.
• Add governance documents docs/testing/COVERAGE_STATUS.md and docs/testing/CRITICAL_INVARIANTS.md, update TESTING.md to require the measurable artifacts, and update docs/README.md to expose the new testing/ documentation surface.
• Include a generated snapshot artifacts/testing/test_inventory.json (timestamped) and minor cleanup to the inventory script for deterministic output.

Testing

• Ran make test-inventory which executed the generator and wrote artifacts/testing/test_inventory.json and docs/testing/TEST_MATRIX.md successfully.
• Ran python3 scripts/testing/generate_test_inventory.py directly to validate the script path and outputs, which completed successfully.
• Attempted cargo test -p tests --lib but it failed in this environment due to crates.io network/download errors (CONNECT tunnel failed, response 403), so full Rust test execution was blocked by external network limitations rather than by the changes.
• The generated inventory reports 1247 crate-scoped test entries and 1255 total visible test/property entries in-repo (see artifacts/testing/test_inventory.json).

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Codex Task