system: validate development environment overlay

docs/system-deployment-workflow.md

@@ -1,6 +1,6 @@
 # Fruix system deployment workflow
 
-Date: 2026-04-04
+Date: 2026-04-05
 
 ## Purpose
 
@@ -197,6 +197,32 @@ Important current limitation:
 - `fruix system switch` does **not** yet fetch or copy the candidate closure onto the target for you
 - it assumes the selected closure is already present in the installed system's `/frx/store`
 
+### In-guest development environment
+
+Opt-in systems can also expose a separate development overlay under:
+
+- `/run/current-system/development-profile`
+- `/run/current-development`
+
+Those systems now ship a helper at:
+
+- `/usr/local/bin/fruix-development-environment`
+
+Intended use:
+
+```sh
+eval "$(/usr/local/bin/fruix-development-environment)"
+```
+
+That helper exports a development-oriented environment while keeping the main runtime profile separate.  The validated Phase 20.1 path currently uses this to expose at least:
+
+- native headers under `usr/include`
+- FreeBSD `share/mk` files for `bsd.*.mk`
+- Clang toolchain commands such as `cc`, `c++`, `ar`, `ranlib`, and `nm`
+- `MAKEFLAGS` pointing at the development profile's `usr/share/mk`
+
+This is the current Fruix-native way to make a running system suitable for controlled native base-development work without merging development content back into the main runtime profile.
+
 ## Deployment patterns
 
 ### 1. Build-first workflow