Adopt the glossary's vocabulary in the mutable design docs

"control plane" -> controller and "substrate" -> foundation throughout
03-DESIGN, 00-META and the README, with 06-the-control-plane.md and
07-the-substrate.md renamed to 06-the-controller.md and 07-the-foundation.md.
The immutable 02-DECISIONS records keep their original wording (and links to
them are unchanged) — a term retired here may still appear there, which the
glossary explains how to read.

Claude-Session: https://claude.ai/code/session_01D6qtiYU3P9jk3pnAXyAFyx

03-DESIGN/01-to-be/02-scenario-declaration.md

@@ -205,7 +205,7 @@ machines:
 
 place:
   all: [host]
-  anchor: [substrate]
+  anchor: [foundation]
 
 snapshot: raised
 ```
@@ -334,12 +334,12 @@ than a fork.
 # bootstrap — tiers 0 and 1
 place:
   all: [host]
-  anchor: [substrate]
+  anchor: [foundation]
 
-# full — adds a control plane, a forge, and a module under test
+# full — adds a controller, a forge, and a module under test
 place:
   all: [host]
-  anchor: [substrate, control, forge]
+  anchor: [foundation, control, forge]
 module: a-web-service
 assert:
   - the service answers on its published name
@@ -524,7 +524,7 @@ machines:
 
 place:
   all: [host]
-  anchor: [substrate]
+  anchor: [foundation]
 
 snapshot: raised
 ```