Repair final two duplicate-answer content defects

[IXI-2773]

Summary

• Corrects exactly sc900_p2_q001 and sc900_p3_q071, the two duplicate-correct-answer defects identified by the completed 454-question audit.
• Preserves prompts, keys, objectives, tested decisions, question IDs, and question count.
• Replaces only the duplicate distractor with Encoding and synchronizes the affected choice feedback.
• Adds governed correction authority, deterministic builder, currentness/spec/review/manifest evidence, and progress/session reset tests.
• Does not activate the successor bank and does not modify CAND participant state.

Validation

• correction suites: 37 tests PASS
• explanation suites: 78 tests PASS
• CAND suite: 188 tests PASS
• full repository suite: 1685 tests PASS
• Ruff / Black / mypy: PASS
• git diff --check: PASS
• production CAND ledger unchanged at SHA-256 b41627840a79b06dbfc12d7b831d838e16a21dd2435e39fd60a59e3b8b5c3ffc, 2032 bytes

Successor identity

• source SHA-256: a3a8b811b3ce14e76f871333fd3a1ba262afda1b2da39fde98ccf6fde6dd522f
• target SHA-256: f97f76591ebb41dd1b92ca5623c6f040b2d9a75ca6f7f1d5b0ebd9adf371581f
• target content fingerprint: 76a2ca2d8779e6e0b422f9192868e19ac1f7e0e0543e555a80a8dde9be14f407

Do not merge or activate until exact-head review.