V2: Envelope Inspector

[afogel]

Master doc: docs/shaping/acs-reference-impl-slices.md §V2 — authoritative for this slice's scope.

Demo: Watch the ACS request and response JSON stream live while you work.

Blocked by: #2
Plan: docs/superpowers/plans/2026-08-09-v2-envelope-inspector.md

Amendments

• §V2 gains decisions P1–P5, three watch-fors, and two scope-added rows; risk row 9 and D10 added — 7e4e857
• §V2's "records the wire verbatim" watch-for corrected: S6 records the JSON value the Guardian parsed, unmodified — not the bytes. Risk row 10 added (synchronous tap writes on a single-threaded server) — bac606d