docs: name the specific daemon instead of the vague 'backend'

Per mara's ruling on hyperhive#4041 (Microsoft.Avoid): 'backend' never says
which system it means. Fixed all 21 fixable hits to name the actual daemon
(mostly hive-c0re; one hive-agent instance and one hive-matrix-daemon one,
both confirmed against surrounding context, not guessed). Also tightened
Weston's own 'VNC backend'/'RDP backend' plugin references to their literal
upstream names (vnc-backend/rdp-backend) rather than dropping the accurate
term entirely, and reworded one unrelated Microsoft.Avoid hit ('and so on').

Left alone (flagged on the issue, not guessed): docs/turn-loop/config.md's
3 remaining hits match the actual nix option name (backendEnvironmentFile)
and describe a genuinely pluggable concept (which LLM API provider), not
one specific internal system.

docs/trust-boundary/boundary.md

@@ -26,7 +26,7 @@ always on), the gateway proxies all operator-facing traffic, and
 ## Design rule
 
 **Operator-authority actions never get a per-agent-socket entry
-point.** They live on the core backend.
+point.** They live in hive-c0re.
 
 Worked example — destroying or rebuilding a container is a
 `POST /api/{destroy,rebuild}/{name}` on the core dashboard, _never_ a