feat: add pluggable island selection and population strategies (#464)

[Mqw7373]

Summary

• Add an optional island selector with read-only island state; preserve default scheduling.
• Add decision-returning population hooks for admission, cell replacement, archive/eviction, and migration. ProgramDatabase validates and applies all mutations.
• Document the extension points and add focused selector/strategy tests.

Closes #464.

Validation

• python -m unittest tests.test_island_selector tests.test_population_strategy -b -q: 25 passed.
• python -m unittest discover tests -b -q on Windows: 455 run, 1 failure and 3 errors. The failures are in YAML temp-file cleanup (2), Windows os.kill(pid, 0) behavior (1), and absolute-path normalization (1). These have not been verified on Linux.
• Black, isort, and diff checks passed for the committed changes.

[CLAassistant]

All committers have signed the CLA.

[Mqw7373]

Superseded by #493, which contains the reviewed changes as a single commit.