[rc1+] sdc detector: document threshold-lowering for sub-0.5pp SDC

[trilamsr]

PR #344 ships pattern-13 (silent data corruption) with default AccuracyDropThreshold=0.005 (0.5pp). Spec asserts "typical SDC regressions land between 0.5pp and 3pp" but provides no citation. Real-world edge cases (single-bit weight flip in small model, transient single-batch corruption in large training) can produce sub-0.5pp regressions that the detector silently skips.

Asks:

1. Document in spec + ATTRIBUTES.md when operators should lower threshold below 0.5pp (currently only "raise" guidance exists).
2. Add citation or empirical basis for the 0.5pp floor.
3. Consider whether default should be lower for high-precision recipes.

Tracks: reviewer finding on PR #344 (SDC detector ships).

Labels: rc1-prep, docs, pattern-13