native-build: introduce executor model

docs/GUIX_DIFFERENCES.md

@@ -294,6 +294,22 @@ Compared with Guix, this is a more explicit split between:
 - a mutable result/staging area for native build execution
 - and the immutable store identities that Fruix treats as the real promoted result
 
+Fruix also now models native-build placement more explicitly as an executor choice.
+
+Current executor kinds are:
+
+- `host`
+- `ssh-guest`
+- `self-hosted`
+
+So instead of treating:
+
+- host-driven builds
+- host-initiated guest builds
+- guest self-hosted builds
+
+as three unrelated architectural forks, Fruix is moving toward one native-build result model with different executor policies attached to it.
+
 ## Where Fruix is intentionally trying to improve on Guix's representation
 
 Fruix is not trying to improve on Guix's core semantics. Guix already got those right.