fix: fail on required output errors

[incendiary]

Summary

• raise an explicit OutputWriteError when required output creation or appends fail
• clear stale actionable output at run start and abort before allocator publication on write failure
• cover allocator open, write, replace, stale-file, and temporary-file failure behavior
• describe the behavior as operational reliability in the roadmap and README

Verification

• Ruff check and format check passed
• baseline: 5 passed
• full suite: 315 passed, 95% coverage for classes and imports
• focused reliability suite: 55 passed
• Gitleaks: no leaks in the commit range
• TruffleHog v3.96.0: zero verified or unverified secrets in the commit range

Live acceptance

• production CLI completed through the host system resolver
• current AWS, GCP, and Azure catalogues were complete and usable
• both public inputs resolved; 45 attribution records produced
• 20 allocator targets validated against the v1 JSON Schema
• no allocator temporary file remained

Direct public-resolver transport remains delegated through docs/EXTERNAL-ACCEPTANCE.md because this execution environment does not permit that network path.