[PHP] fix: reject unsupported mremap flags

[brandonpayton]

Why

mremap(2) accepts flags Kandelo does not implement. The wasm libc import ABI for mremap passes only the first four Linux syscall arguments, so MREMAP_FIXED (which needs the fifth new_address argument) cannot be honored, and MREMAP_DONTUNMAP has Linux-specific aliasing semantics the kernel does not implement. Silently accepting those flags meant Kandelo would fall through to an ordinary in-place remap — promising behavior it does not provide and risking corrupt mappings.

Returning a deterministic error is the correct kernel contract: it keeps the implemented surface precise and testable. PHP's memory-management tests probe exactly these mremap edge cases, so the fix belongs in the kernel rather than in a PHP shim.

What

• crates/kernel/src/syscalls.rs: define SUPPORTED_FLAGS = MREMAP_MAYMOVE in sys_mremap and return EINVAL for any other flag bits, before attempting the remap. Adds a test that MREMAP_FIXED is rejected with EINVAL and leaves the original mapping intact.
• crates/kernel/src/wasm_api.rs: update the kernel_mremap doc comment to state that in-place resize and MREMAP_MAYMOVE are supported while other flags are rejected.

Provenance

Supersedes fork-headed PR #730; identical head commit 978e08195c21b5e48cf71f35bd1164c173ba7de7, re-pointed onto an Automattic/kandelo branch. No code changed from #730.