Cross-platform embedding of HVM4

[lancejpollard]

It would be wonderful to make HVM4 compilable as an embedded library on WASM, iOS, Android, and Linux/macOS/Windows by adding minimal #ifdef guards to the system abstraction layer. Zero changes to the core evaluation engine.

Made 5 independent PRs/updates to do this, each on its own branch, would be great if you could take a look:

1. Configurable HEAP_CAP and MAX_THREADS - Allow tuning for constrained environments (WASM linear memory, mobile address spaces).
2. Platform-abstract mmap/munmap - Compile on platforms without POSIX mmap (WASM, embedded).
3. Guard dlopen/dlsym - Compile on platforms without dynamic linking (WASM, iOS App Store).
4. Guard fork/execvp in AOT builder - Compile on sandboxed platforms that prohibit process spawning (WASM, iOS, Android).
5. Add lib.c library entry point - Expose HVM4 internals as real linker symbols so external code can call into the runtime.

Scope

• ~50 lines changed across 5 files, plus 1 new file (lib.c)
• All guards default to existing behavior
• All existing tests pass unchanged
• Only the system abstraction layer is touched
• Core evaluation is completely untouched