docs(k2-aic): add K2 failure boundary study artifacts

[mvillmow]

Summary

• add epic-scoped K2 failure-boundary tables under docs/k2-aic-exhaustive-study/data/138_*
• add boundary figures, including a failure-vs-unsupported ratio pointcloud, under docs/k2-aic-exhaustive-study/figures/138_*
• add the narrative entrypoint and section prose under docs/k2-aic-exhaustive-study/sections/138-*

Commit mapping

• #139 4102848f9b docs(k2-aic): add failure boundary tables
• #140 7ae5b66928 docs(k2-aic): add failure boundary figures
• #141 b69a82cb2c docs(k2-aic): add failure boundary narrative

Notes

• based on origin/main
• self-contained and independent of #181
• raw/private source catalogs remain outside GitHub; only safe derived artifacts are checked in here
• reproducibility scripts remain out of scope for this PR and are tracked separately in RL360 issue #182

Verification

• verified the merged derived dataset contains 5,100 agg/disagg boundary cells
• verified class counts: supported_both=4560, unsupported_disagg=285, failed_both=255