fix(io): make io worker cpu pinning opt-in via MORI_CORE_OFFSET

[isytwu]

Follow-up to #358 (cpuset-aware worker pinning).

Changes

• IO worker CPU pinning is now opt-in: workers are only pinned when
  MORI_CORE_OFFSET is set. When it is unset, workers inherit the process
  cpuset and the scheduler places them (matches the default no-explicit-pin
  behavior used elsewhere, e.g. SGLang's opt-in SGLANG_SET_CPU_AFFINITY).
• Parse MORI_CORE_OFFSET via mori::env::GetInt instead of std::stoi,
  so an invalid value is ignored (treated as unset) instead of throwing and
  terminating the worker thread. Added a small signed-int helper
  (ParseInt / GetInt) alongside the existing ParsePositiveInt /
  GetPositiveIntOr in env_utils.hpp.

Why opt-in

Implicitly pinning every worker to allowed[0..n] can collide with other hot
threads sharing the same cpuset (e.g. the SGLang scheduler/compute threads).
Pinning is a tuning decision, so the default is now neutral; operators opt in
when they have a deliberate core layout.

Test

On 086/087 under taskset -c 8-15 with --num-worker-threads 4 --disable-chunking:

• MORI_CORE_OFFSET=0: workers bound to cores 8-11 (within the allowed set).
• unset: no pinning, workers run on scheduler-chosen allowed cores (e.g. 11-14).
• MORI_CORE_OFFSET=abc (invalid): no crash, treated as unset, workers run normally.
  Plus a unit check of GetInt parsing (valid / 0 / negative / garbage / overflow).