docs: C ABI migration campaign plans, handoffs and roadmap

Complete documentation set for the facade migration (B1-R6) and beyond:
facade-migration-roadmap, handoffs v3-v6, R5 guides, R6 cleanup plan,
R7 pure-ABI plan, long-term plans (riir/ai-agent/gtest/ui-redesign),
updated Chinese README draft, UI design mockups, and the Google Test +
struct-typedef rules in CONTRIBUTING.

CONTRIBUTING.md

@@ -22,6 +22,7 @@ submitted should abide by the following standards:
   * Documentation comments should use **Javadoc-style** (`/** ... */`) where appropriate.
 * Naming rules (enforced by `readability-identifier-naming` in `.clang-tidy`):
   * Types (`class`, `struct`, `enum`, type aliases, template parameters): `PascalCase`
+  * `typedef` of structs is permitted (e.g. the opaque-handle pattern `typedef struct OakEngineNode OakEngineNode;`); struct typedefs follow `PascalCase`
   * Functions, variables, member variables: `snake_case`
   * Private/protected members: trailing underscore, `class_member_variables_`
   * Constants and enum values: `snake_case` (e.g. `k_dry_run_interval`, `k_linear`); `ALL_CAPS` is reserved for macros — save the fear for things that are actually dangerous
@@ -30,5 +31,6 @@ submitted should abide by the following standards:
   * Namespaces: short `snake_case`
   * Getters: same name as the private member without the trailing underscore (`foo_` → `foo()`); setters: `set_foo()`
   * Exception: Qt and third-party (e.g. OpenFX) virtual overrides and framework callbacks keep their original names (`paintEvent`, `getParams`, ...) — renaming them would break the override
+* Tests are written with **Google Test** (`TEST`/`TEST_F`/`TEST_P` + `EXPECT_*`/`ASSERT_*`). Do not add hand-written test `main()`s, raw `assert()`-based test files, or custom test macros/frameworks. CTest stays the runner only — register cases through `gtest_discover_tests()`; use `GTEST_SKIP()` for environment-dependent cases (GPU, missing codecs) instead of relying on crashes or timeouts..
 * 100 column limit (where it doesn't impair readability)
 * Unix line endings (only LF no CRLF)