Match platform bindings to the C ABI

[elcritch]

Wayland and GLX pointer declarations can use the wrong C calling convention on 32-bit targets, and pointer-sized Vulkan surfaces lose their upper 32 bits. This follows the merged #56 and corrects the bindings and their callers.

• Bind Wayland display/proxy functions with native pointer signatures; pass .raw from Siwin callers.
• Preserve public EGL bool results using small wrappers around private cuint bindings. The dynamically loaded function pointer must return unsigned int; declaring it as bool does not insert a conversion. The wrappers explicitly compare the full result with zero.
• Correct GLX pointer and swap-interval signatures while preserving existing call patterns.
• Use uint64 for Vulkan surface handles on every architecture, including platform bindings and the public API; the C header uses uint64_t.

This replaces elcritch#7 and addresses both review comments there. Direct Wayland callers now pass raw pointers, and Vulkan callers receive a 64-bit integer. EGL callers retain boolean checks; availability and teardown checks use the private native entry points.

Validation on FreeBSD amd64 with Nim 2.2.10 and the existing X11 display (AMD Radeon 780M):

• atlas-run tests _dynlib_loading passed with the default memory manager and --mm:refc. Includes full-width integer-to-bool regression coverage, API type checks, native EGL failure results, and a real EGL display lifecycle.
• atlas-run tests _opengl.nim _offscreen _vulkan _x11_api _event_loop passed (5/5), including live Vulkan rendering and X11/XCB queries.
• nim check --os:linux --path:src src/siwin.nim passed for amd64; nim check --os:linux --cpu:i386 --path:src tests/t_dynlib_loading.nim passed for i386.
• C header syntax and 8-byte Vulkan surface assertion passed; git diff --check passed.

GitHub CI passes on Linux, macOS, and Windows. The local i386 validation is compile-time only; local live-display checks used X11. Downstream integration is tracked in Merenda #138.