[Fix] Validate num_sms before communication context initialization

[morluto]

Fixes #7.

Summary

Validate num_sms before allocating the communication context or compiling
communication kernels.

Explicit values must be positive integers no greater than the current CUDA
device's physical SM count. When omitted, num_sms now resolves to
min(32, device_sm_count).

num_sms is None ----------------> min(32, device SM count)

explicit num_sms
      |
      +--> positive integer? ---- no --> TypeError / ValueError
      |
      +--> <= device SM count? -- no --> ValueError
      |
      `-------------------------- yes -> create context

Problem

MoonEP exposes num_sms as the number of SMs assigned to communication, but
previously accepted any positive integer. Oversized values reached context
allocation and kernel compilation before producing a configuration-dependent
CUDA failure.

The previous fixed default of 32 could also be unsuitable for devices with
fewer than 32 physical SMs.

This change establishes a conservative MoonEP API bound. It does not attempt
to calculate each compiled kernel's occupancy-dependent cooperative launch
limit.

Change

• Reject booleans and non-integer values with TypeError.
• Reject zero and negative values with ValueError.
• Reject explicit values greater than the current device's SM count.
• Resolve None to the smaller of 32 and the device SM count.
• Document the resolved default and upper bound.

Validation

• python3 -m pytest -q tests/test_api_validation.py — 7 passed.
• python3 -m py_compile moonep/api.py tests/test_api_validation.py — passed.
• Coverage includes malformed values, oversized values, the exact device
  limit, and default resolution on a smaller device.