[flyingdoubleG]

Problem

ProgramDatabase.migrate_programs (default ring migration) picks each island's migrants while it is already moving the previous islands' migrants:

1. Island i's best programs are copied into island i+1 (and i-1) before island i+1 is processed.
2. When island i+1 is processed, the fresh copies (metadata["migrant"] = True) compete for its top max(1, int(n * migration_rate)) slots. _migrate_one skips migrants, so those slots are wasted.
3. Worse, the incoming copies can displace island i+1's own programs from their MAP-Elites cells before they get a chance to migrate.

The result depends on island order, and good programs on later islands may never leave their island. test_migration_ring_topology already notes this as "a known limitation of the current implementation that processes islands sequentially while modifying them, causing interference between migration rounds."

Repro: 3 islands, migration_rate=0.1, one program per island with scores 0.9 / 0.5 / 0.3 in distinct cells, then one migrate_programs() call:

main:     island 0: ['a']   island 1: ['a', 'b']   island 2: ['a', 'c']
this PR:  island 0: ['a', 'b', 'c']   island 1: ['a', 'b', 'c']   island 2: ['a', 'b', 'c']

On main only the top program spreads; b and c never migrate.

Fix

• Plan all moves from the pre-migration population first, then apply them.
• Exclude existing migrant copies from migrant selection, since _migrate_one would skip them anyway.

The pluggable population_strategy.migrate path is unchanged.

Tests

New test_every_island_migrates_its_own_best covers the scenario above. It fails on main and passes with this change. tests/test_island_migration.py, test_migration_no_duplicates.py, test_island_isolation.py, test_database.py and test_population_strategy.py all pass: 66 tests, CPU.

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