G-117: record the embed-then-read-back emission design (single-source build_id)

family_artifacts.tcl runs after kits are built, so the embed lands at kit-family
staging and emission mounts each kit exe to READ the embedded record back -
sidecar fields derive from it (identical build_id by construction; emission
inherently verifies the embed exists). sha1/size stay sidecar-only; embedded
filename namespaced per the G-025 reconciliation.

Assisted-by: harness=claude; primary-model=claude-fable-5; api-location=anthropic.com

goals/G-117-self-describing-runtimes.md

@@ -83,8 +83,23 @@ hosts without a zip reader.
   (current local r2 is pre-v1 and unpublished; published r1 stays immutable) ->
   family_check -> only then punkbin r2 publication + defaults.txt flip (G-103
   follow-through), with G-119 fixture-server verdict tests after that.
-- Remaining for acceptance: everything in the Acceptance line (no implementation
-  landed yet).
+- 2026-07-24 implementation design (recorded before staging work; from reading
+  tools/family_artifacts.tcl): family_artifacts.tcl runs AFTER kits are built (copies
+  finished exes, hashes them), so the embed happens at the kit-family staging step and
+  emission READS IT BACK: family_artifacts mounts each kit exe (tcl::zipfs::mount on
+  the file) and loads the embedded record, deriving the sidecar's shared fields
+  (build_id, origin, packager, schema...) from it - single source of truth, identical
+  build_id by construction, and every emission run inherently verifies the embed
+  exists (fail if absent). Staging generates build_id per member and writes the
+  embedded record WITHOUT sha1/size (finished-binary facts stay sidecar-only).
+  Embedded record filename namespaced per the G-025 reconciliation note.
+- Remaining for acceptance: locate/extend the kit-family staging step (embed write),
+  family_artifacts v1 fields + mount-read derivation + '#' field comments +
+  -Doriginurl/-Dpackager plumbing, family_check embedded-record verification,
+  punk-runtime 'info' action (ps1 + bash + cmd rewrap; zip central-directory read,
+  cooperative-probe fallback documented), punkbin AGENTS.md schema v1 docs, suite
+  rebuild + r2 re-emission + wrap trial (also verifies the G-118 items 1+7 patch rev
+  behaviour on rebuilt kits).
 
 ## Notes