Complete fixture execution and corrected answer contracts

[ma7dev]

Changes

• Add fixture-tools-v1: bounded virtual reads/writes, evidence transcripts, usage and timing; no host filesystem or shell.
• Add strict scoring for four documented Absher answer corrections.
• Pin the next public release and document harness conditions, limits and scoring.

Verification

25 tests and Ruff pass, including file-write evidence, path traversal, read-only inputs, output limits, turn exhaustion and strict answer keys.