v2.1.2 - Streaming safety fixes

Fixed

• Preserve the payload-fingerprinted ambiguous request lock when a streamed generation completes with only partial_failed events, preventing an automatic retry without billing certainty.
• Track occupied stream target indices and reject conflicting reuse of the same image_index, even when --force is enabled.
• Assign indexless stream results to the first unoccupied target and validate usage.generated_images against unique saved targets.

Documentation and validation

• Updated the Lite contract, execution-flow documentation, and bilingual README wording.
• Added regressions for mixed indexed/indexless events, conflicting reused indices, force-overwrite protection, and all-failed stream billing locks.
• Local validation: 165 passed, 1 skipped, 98 subtests passed; compileall, pip check, release-tag verification under UTF-8 and cp1252, and git diff --check all passed.
• No real Ark request was issued.