docs(engine): pick ENOSPC injection mechanism for spill tests (SPL-33.a)

Evaluate five candidate ENOSPC injection mechanisms (tmpfs size-cap,
fallocate filler, mock writer, failpoints crate, fuse-mt) against CI
compatibility on the Linux musl / macOS / Windows tiles and against
the SPL-32 audit's site inventory. Recommend a userspace mock writer
for the unit-test layer plus a Linux-gated fallocate filler for one
real-kernel integration test, and pin the chosen mechanism's API
surface in pseudo-code so SPL-33.b has a concrete target.

Cross-references docs/design/spill-fs-error-audit.md (SPL-32) for the
syscall site inventory and the SpillError::PriorSpillsLost wiring
landed via SPL-35/36/37 (#4749).

Author: oferchen