docs: v2.1 - address all reviewer feedback on test strategy

Key changes based on 3 rounds of review:

VECTORS:
- Use semantic error codes (ERR_EMPTY_INPUT) not exception names
- Add case IDs and tags for traceability
- Add JSON Schema for vector validation
- Add python_verified/typescript_verified dates
- Add COVERAGE.md tracking file

CRITICAL FIX:
- DON'T refactor Python to ReportResult - test AS-IS
- Divergent implementations, convergent vectors

SNAPSHOTS:
- Add SnapshotSanitizer for timestamps/IDs/durations
- Document snapshot location and review process

SHELL SCRIPTS:
- Add shellcheck as "third way" mitigation
- Add E2E smoke test option

CI:
- Add pytest-repeat, pytest-split to deps
- Fix diff-coverage to handle no-Python-changes case
- Cache pip download dir, not .venv

PROCESS:
- Add Definition of Done per phase
- Add Stop Condition for ambiguous behavior
- Add Rollback Plan
- Add phase dependencies
- Note mutation testing as optional/monthly

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: Nik Samokhvalov