docs: release & packaging spec + ADR 0007 amending 0004 (map #33, task #42)

- docs/spec/release-packaging.md: decision-complete spec — compat matrix,
  Gitea 1.27.1 registry channel, nfpm toolchain, signing/key policy,
  release mechanics, package layout/ownership, maintainer-script
  contracts, initial config, make-install migration runbook.
- docs/adr/0007: package delivery amends ADR 0004 (delivery/ownership
  only; runtime semantics inherited verbatim). 0004 status updated.
- docs/research/: toolchain, gitea-registry, obs findings merged from
  research branches (assets of map tickets #34/#35/#36).
- CONTEXT.md: Release + Rollback glossary terms (ticket #43).

CONTEXT.md

@@ -80,6 +80,14 @@ _Avoid_: Uptime, sample count
 One scheduled or on-demand execution of the collector that interrogates the drive and extends the observation history.
 _Avoid_: Poll, daemon tick
 
+**Release**:
+A published version of Fenris: a version tag, its packages in the channel, and its human-readable change notes, all together; a bare tag is not one.
+_Avoid_: Tag, upload, build
+
+**Rollback**:
+Returning to an earlier release by restoring an observation-store snapshot and then installing that release; installing an older package over a newer store is unsupported.
+_Avoid_: Downgrade, version pinning (as a promise)
+
 **Deliberate disable**:
 A monitoring pause made through Fenris's own control path, closing the monitoring period so the paused time is excluded from the usage habit.
 _Avoid_: Manual stop, service stop