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/01-end-to-end-testing.md

@@ -40,13 +40,13 @@ not the first one built** ([ADR 0016](../../02-DECISIONS/0016-the-lab.md)).
 
 | | **Bootstrap scenario** | **Full scenario** |
 |---|---|---|
-| Contains | virtual machines, the host binary, a pinned substrate bundle | a complete mesh: forge, coordinator, delivery, modules |
+| Contains | virtual machines, the host binary, a pinned foundation bundle | a complete mesh: forge, coordinator, delivery, modules |
 | Verdict from | what the host reports about the state it reconciled | a pipeline result ending in verify |
-| Exercises | the node host and the substrate | the control plane and everything above it |
+| Exercises | the node host and the foundation | the controller and everything above it |
 | Exists to | **develop the mesh** | **test what runs on it** |
 
 The bootstrap scenario is a **strict subset**: same virtualisation, same networking, same
-lifecycle — it simply stops before a control plane exists. Everything from *"Where this sits in
+lifecycle — it simply stops before a controller exists. Everything from *"Where this sits in
 the way work happens"* onward describes the full scenario, and applies once there is a
 coordinator to describe.
 
@@ -343,7 +343,7 @@ from the existing system has run against any of it yet.
 
 ---
 
-## The substrate
+## The foundation
 
 ### A node is a system container