Addresses the review of 2f2d7f1d + 5c30bfe0:
- arm_timer: add the null-`current` guard its siblings (sleep/suspend_self/
block_on_fd) all have. Armed from the bare scheduler context it stored a
`fiber = null` Timer that segfaulted `wake` on fire; now it aborts loudly.
- spawn_raw: bail loudly on a null `entry` (a null fn-ptr jumped to 0x0 once
the fiber ran), and document the `arg` lifetime contract (must outlive the
fiber's first run, not just the spawn_raw call).
- poll: abort if io-waiters are pending (a `poll`-only driver can't progress
fd-bound fibers — previously returned -1, indistinguishable from quiescent),
and document `deadline_ms` as reserved/intentionally-unread for the
virtual-time single-step (not a silently-dropped arg).
- fib_dispatch: replace the now-stale comment (which still claimed fibers run
under `__sx_default_context` and "do not inherit a caller-scoped allocator")
with the Phase 0 reality + the dctx lifetime contract: every capability in
the spawn-time context (allocator/io/data) must outlive the fiber.
Behavior-preserving (full suite 828/0); the new aborts only fire on misuse.
da7dd1f1e7