lvgl host: fix platform_set_embed_parent signature

[defiantnerd]

A developer configuring with -DNEUI_WITH_LVGL=ON hit platform_set_embed_parent as an undefined symbol at link time. The seam wasn't missing — it was declared with the wrong parameter type.

hosts/crossplatform/platform.h:62 declares:

void platform_set_embed_parent(Session*, uint32_t, void* native_parent);

platform_lvgl.cpp defined the third parameter as unsigned long. That's a different overload, and on MSVC x64 unsigned long is 32-bit so it can't even collapse into the pointer type by accident. The void* version that widgets.cpp:1987 actually calls was never defined. Every other platform layer (win32, macos, linux, ios, null) already used void*.

Changes

• Match the header signature, and record the value into wd->embed_parent the way the null host does, so the public NEUI_API_EMBED state round-trips. LVGL owns the whole display, so nothing acts on it: there is no foreign native parent, event_fd stays -1, pump_and_tick stays a no-op.
• Add the embed seam to the stubbed-or-absent list in docs/host-lvgl.md — "might not be needed in LVGL, but the host must still provide it" was itself a doc gap.

Interface audit

Diffed every platform_* seam declared in platform.h against the definitions in all six platform layers (return type + parameter types). This was the only real mismatch. platform_retained_tree_changed / platform_retained_widget_invalidate are LVGL-only seams (guarded by NEUI_PLATFORM_LVGL in the header) and correctly defined only there.

Verification

Not built: platform_lvgl.cpp compiles only on Windows with -DNEUI_WITH_LVGL=ON, and this was authored on macOS. The change is confined to that one file, and get_widget / embed_parent are both already used throughout it — a Windows LVGL build should confirm the link error is gone.