feat(kernel): add scheduler wakeup channel

[MikiEEE]

Summary

• add an opaque kernel-owned cross-thread scheduler wakeup channel
• implement Unix support with a nonblocking socket pair and coalesced notifications
• bound interrupted send and drain attempts
• preserve observable EOF or retryability across notification failures
• reject invalid socket-pair endpoints before consumer startup and clean partial construction exactly once
• keep generic MicroPython support explicitly disabled unless a port can honor the complete contract
• verify repeated wakeups interrupt an already-entered persistent wait

Review

Independent adversarial review found two medium issues: incomplete endpoint preflight and a cross-thread test that could notify before the scheduler blocked. Both were fixed and targeted re-review approved. The critic pass additionally hardened invalid send-result handling. No blocking findings remain.

Validation

• 134 unit tests passed
• focused kernel and runtime wakeup tests passed
• package, test, and demo compilation passed
• git diff --check passed
• local Pyright, coverage, and PEP 517 build tooling were unavailable; GitHub CI is authoritative for those gates

Residual risk

Thread scheduling remains platform-dependent, though the test now synchronizes notification from the registered persistent-wait path.

Closes #37