fix: fill inspiration slots in small islands

[liuzhengyang699]

When an island has no more programs than the requested inspiration count, _sample_inspirations skips its diversity and random-fill stage. For example, with a parent and two alternatives, requesting five inspirations returns only the elite instead of both alternatives. This unnecessarily narrows the code examples available to the next generation.

Run the fill stage whenever inspiration slots remain. Existing parent exclusion, uniqueness, elite priority, and island isolation still apply; an undersized island simply contributes its available alternatives.

Validation: new regression cases cover an island equal to and smaller than the requested count, a one-slot request, and a parent-only island. The small-island cases fail before the fix. 16 focused island sampling tests and the repository’s isort/Black pre-commit hooks passed. No model calls were made.

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