fix: use island-aware inspiration sampling in sample_from_island

[pathakanshika144-afk]

Summary

Fixes #437.

sample_from_island() selects the parent with the same exploration/exploitation/weighted strategy as sample(), but samples inspirations with plain random.sample() when the island is non-empty. Only the empty-island fallback (self.sample()) uses _sample_inspirations(), which applies the island-best + elite + feature-cell diversity strategy. As the issue notes, _sample_inspirations() should be used in all cases.

Change

• Add island_id: Optional[int] = None to _sample_inspirations(). When provided, it overrides parent.metadata["island"]; when omitted, behavior is unchanged.
• sample_from_island() now calls self._sample_inspirations(parent, n=num_inspirations, island_id=island_id) instead of random.sample(other_programs, ...).

Why an explicit island_id

_sample_inspirations() resolves the island via parent.metadata.get("island", self.current_island). Reusing it naively would reintroduce two problems:

1. Thread safety — sample_from_island() is documented as thread-safe and does not touch shared state. The current_island fallback would reintroduce a shared-state read for parallel workers.
2. Archive fallback isolation — _sample_from_archive_for_island() falls back to any archive program when the requested island has none, so the returned parent's metadata["island"] may differ from the requested island_id. Relying on parent.metadata would pull inspirations from the wrong island, breaking the genetic isolation that test_sample_from_island_returns_from_correct_island asserts.

Passing island_id explicitly closes both gaps.

Compatibility

• sample() is unchanged — it calls _sample_inspirations(parent, n=...) without island_id, preserving the existing parent.metadata -> current_island resolution.
• parent_island %= len(self.islands) is added defensively; sample_from_island() already wraps island_id at entry, so this is a no-op for the new call site but guards future callers.

Tests

New tests/test_sample_from_island_inspirations.py (3 tests):

• test_sample_from_island_reuses_strategy_aware_inspiration_sampler — verifies delegation to _sample_inspirations(parent, n=..., island_id=island_id).
• test_explicit_island_overrides_parent_metadata_for_inspirations — parent from island 0, request island 1; inspirations come from island 1, not the parent's island.
• test_zero_inspirations_is_forwarded_without_random_sampling — num_inspirations=0 is forwarded without random sampling.

Existing tests/test_sample_from_island_ratios.py (8 tests) still passes, including test_sample_from_island_returns_from_correct_island (inspirations stay on the requested island) and test_single_program_island (single-program island returns no inspirations).