docs(P-038): ratify boundary transition witness experiment

[PhysShell]

What changed

• adds P-038 as an accepted experiment, with product adoption explicitly out of scope
• freezes the edge/site/runtime identity model and observability-bounded negative claims
• records the calibration → funding → adjudication ladder, matched case/control design, primary estimand, and mechanical POSITIVE / NEGATIVE / INCONCLUSIVE decision rule
• adds P-038 to the proposal index

Scope guard

No implementation, runtime collector, ARCH finding code, architecture-gate behavior, rules.yaml change, baseline-ratchet change, or product adoption is introduced.

Validation

Docs-only diff against main: one new proposal plus one index row. P-038 remains downstream of P-023/P-032 boundaries and explicitly cannot alter Own.Arch verdicts.

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