docs: update architecture docs for addon-first model (#255)

ARCHITECTURE.md:
- Replace exec()-based bootstrap flow with manifest-driven loader diagram
- Add addon lifecycle section (scan → parse → validate → resolve → load → register)
- Add SDK and C++ scaffold to source layout
- Document load order: sdk (0) → ztools (50) → silo (60)

INTEGRATION_PLAN.md:
- Add Layer 4 (Addon SDK) between bootstrap and addons (now 5 layers)
- Update Layer 3: now hosts both Python loader AND C++ scaffold
- Add Phase 1.5 (SDK) and Phase 1.75 (C++ scaffold) as DONE
- Update Phase 4 (theme): colors centralized in SDK YAML palette
- Update Phase 6 (build): now DONE with CMake install rules for all addons
- Add design decisions #7 (SDK as adaptation layer) and #8 (manifest-driven loading)

README.md:
- Update addon integration section: manifest-driven loading, SDK wrappers
- Add mods/sdk/ to project structure tree
- Update Create module description to mention C++ scaffold

Closes #255.

README.md

@@ -112,7 +112,9 @@ Addons extend this through the Python API:
 
 ### Addon integration
 
-Both ztools and Silo are loaded by the `src/Mod/Create/` bootstrap module, which executes their `Init.py` and `InitGui.py` files at startup. Neither requires the user to switch workbenches. Instead, they register commands and use the editing context system to make those commands visible in the appropriate situations.
+Addons in `mods/` are loaded by a manifest-driven loader (`src/Mod/Create/addon_loader.py`). Each addon provides a `package.xml` with `<kindred>` extensions declaring version bounds, load priority, and dependencies. The loader resolves dependencies via topological sort and loads addons in order: **sdk** (0) → **ztools** (50) → **silo** (60).
+
+Addons call platform APIs through the **kindred-addon-sdk** (`mods/sdk/kindred_sdk/`) rather than `FreeCADGui.*` internals directly. The SDK provides stable wrappers for editing contexts, theme tokens, FileOrigin registration, and dock panels.
 
 ztools uses a `WorkbenchManipulator` plus `injectEditingCommands()` to add its tools to PartDesign, Assembly, and Spreadsheet contexts. Silo registers an overlay that adds the "Silo Origin" toolbar whenever the active document contains Silo tracking properties (`SiloItemId`, `SiloPartNumber`).
 
@@ -179,13 +181,14 @@ create/
 │   │   ├── Stylesheets/       #   QSS theme files
 │   │   └── PreferencePacks/   #   Theme and preference configurations
 │   ├── Mod/                    # FreeCAD modules (PartDesign, Assembly, Sketcher, ...)
-│   │   └── Create/            # Kindred bootstrap module -- loads ztools and Silo
+│   │   └── Create/            # Kindred Create module (Python loader + C++ scaffold)
 │   └── 3rdParty/              # Vendored dependencies
 │       ├── OndselSolver/      #   Assembly constraint solver (forked)
 │       └── GSL/               #   Microsoft Guidelines Support Library
-├── mods/                       # Kindred addon modules (git submodules)
-│   ├── ztools/                # ztools command provider
-│   └── silo/                  # Silo PLM workbench
+├── mods/                       # Kindred addon modules
+│   ├── sdk/                   # Addon SDK — stable API contract (priority 0)
+│   ├── ztools/                # ztools command provider (submodule, priority 50)
+│   └── silo/                  # Silo PLM workbench (submodule, priority 60)
 ├── resources/                  # Branding, icons, desktop integration, MIME types
 ├── package/                    # Packaging scripts
 │   ├── debian/                #   .deb build script