Add monthly build corpus and offline CI comparisons

[AdamZ-8113]

Replaces live URL downloads during build comparisons with a saved, validated monthly input corpus. The updater consumes PoB Codes' versioned batch, retains at most 500 unique codes in FIFO order, and publishes the manifest and exact bytes together on build-test-corpus. PR jobs pin one corpus commit and explicit base/head revisions, then calculate identical inputs offline.

Companion provider change: https://github.com/AdamZ-8113/pob-codes/pull/10

The host Python runner uses a pinned Docker environment, bounded parallelism/timeouts, and each revision's own runtime. It preserves ordinary DiffOutput.lua output: metric changes are advisory unless strict mode is requested; failed imports, crashes, empty outputs, and incomplete comparisons fail. Fixed-fixture smoke runs independently of corpus activation. Obsolete downloader/profile/Compose plumbing is removed.

Validation:

• 12 Python tests and the existing 160-test Busted suite passed.
• Five fixed input pairs matched in strict mode on the existing tests-branch runtime.
• Nine input pairs (shared API fixtures, existing fixed fixtures, a current public build, and a synthetic minion build) matched in strict mode on dev 03ae46279c6570facabc5fb65ec5d171edc339fd, completing in 28 seconds locally.
• Controlled stat drift returned advisory success / strict failure; empty output and incomplete runtime initialization failed.

This is deliberately a draft against tests-branch. The 2024 runtime cannot handle the tested modern 3.28 export. A supported runtime, 100/500-distinct-input GitHub-runner benchmarks, provider activation, and default-branch registration are prerequisites for enabling TEST_BUILD_CORPUS_ENABLED. A merge only to tests-branch does not activate GitHub scheduling. The updater uses no API secret and never force-pushes; a competing writer retries on the next daily run.

No legacy corpus migration, custom exporter, output cache, dashboard, or reporting pipeline is included. CONTRIBUTING.md documents the contract, reproduction commands, and provider/CI ownership.