[panshularora]

Summary

Closes #304.

Reject fork --at values outside 0 <= K < len(chain) immediately after reading the source chain, before indexing, reading prepared documents, or creating output. The error includes the valid range.

Valid boundary handling, checkpoints, source chain prefixes and exclusion rules are unchanged. Document the argument contract in simple/README.md.

Verification

• Test-first: negative, chain-length and oversized inputs failed on unchanged upstream with raw IndexError/FileNotFoundError; after the guard they reject cleanly and leave no output.
• Full Python 3.12 CPU suite: 73 passed, 9 subtests passed.
• ruff check simple tests and git diff --check: passed.
• Existing CLI reproduction rerun: valid index succeeds; negative/chain-length inputs reject without partial output.
• Tests cover both first/last eligible boundaries, exact prefix/checkpoint preservation, and existing non-Wikipedia/already-forked rejection.

Limits / review focus

Synthetic CPU chain/checkpoint fixtures, not published trained weights or RTX 5090 replay. This fixes invalid argument handling, not atomic cleanup for other failures (e.g. a missing checkpoint at a valid index). No digest, RNG, optimizer, recipe or training arithmetic changes.

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