Add plan-only proof-carrying context experiment

[ictechgy]

Summary

• add the default-off, high-risk proof-carrying-context experiment as a plan-only surface
• enforce strict proof-unit JSON validation, deterministic readiness output, non-echo boundaries, and explicit provider/protected-zone gates
• synchronize the packaged plugin copy and document one executable canonical command in all six required docs
• add regression coverage for schema, parser, depth/overflow, side-effect, determinism, and runtime/package boundary invariants

Scope boundaries

• no emit, record, or serve runtime
• no new binary, package, version, changelog, manifest, or release-surface change
• actual receipt/content/range/protected-zone verification remains explicitly out of scope

Validation

• python3 scripts/prepublish_check.py — 768/768 passed
• python3 scripts/release_smoke.py — OK
• proof-carrying-context tests — 12/12 passed
• related registry/semantic tests — 31/31 passed
• plugin sync, py_compile, and git diff --check — passed
• independent code review — APPROVE
• independent verifier — VERIFIED