Slice 1: Meson skeleton — kernel links wlroots 0.20 from C++, RMLUi 6.2 vendored

Root meson.build (C++23, WLR_USE_UNSTABLE, ccache-detected) with RMLUi 6.2
as a wrap-file tarball built through the cmake module (no git submodules —
settled decision) and doctest 2.5.2 from wrapdb. kernel unit: extern-"C"
wlr.hpp wrapper (with the C99 [static N] array-param workaround documented
in kernel.md), slice-1 probe contract, doctest suite (1/1 green). host-bin:
composition root printing versions, exit 0. tasks.md slice 1 done.

1 files changed, 19 insertions, 0 deletions

diff --git a/packages/kernel/kernel.md b/packages/kernel/kernel.md
new file mode 100644
index 0000000..e2ee684
--- /dev/null
+++ b/packages/kernel/kernel.md
@@ -0,0 +1,19 @@
+# kernel — package notes
+
+Slice-1 state: a probe surface (`kernel.hpp`) proving C++ ↔ wlroots ↔ RMLUi
+compile/link. Real contracts (extension host, bus, scene/seat glue, ui
+substrate) land from slice 2.
+
+Gotchas the headers can't express:
+
+- **`wlr.hpp` blanks `static` around the wlr includes.** wlroots headers
+  use C99 array-parameter syntax (`float color[static 4]`), invalid in
+  C++. With `static` blanked, `static inline` helpers become `inline`
+  (ODR-merged, safe). Cost: a function-local `static` inside a header
+  inline would silently lose persistence — none exist in our include set;
+  re-audit when ADDING includes to the wrapper.
+- **RMLUi is kernel-private.** `rmlui_dep` is deliberately absent from
+  `kernel_dep` propagation (see meson.build): extensions contribute RML
+  documents + data bindings via the ui substrate, never RMLUi API calls.
+  Do not "fix" a missing-RMLUi-header error downstream by propagating it.
+- Everything runs on the single `wl_event_loop` thread.