fix(algorithms): the differential evolution population floor is the strategy's donor count, not always three (#708)

[wshlavacek]

Fixes #708.

Problem

The 2 differential-evolution strategies (rand2, best2, all2) build each candidate from five distinct parameter sets, but both population floors were hardcoded to three and never consulted de_strategy. new_individual draws five donors with rng.choice(..., replace=False), which raises ValueError: Cannot take a larger sample than population when replace is False whenever the population it draws from (per island) holds fewer than five. The failure is an unhandled exception on the first proposal, not a wrong answer.

Two paths reached it:

• ade: population_size = 4 with best2 cleared the < 3 clamp untouched and crashed when the first result returned.
• island de: population_size = 24 over 8 islands gives 3 per island -- which looks reasonable, draws no warning, and crashes the first island to finish a generation. The message it would have printed ("at least 3 times the number of islands") misinformed.

Fix

min_population is computed once in DifferentialEvolutionBase.__init__ (three for a 1 strategy, five for a 2), right after de_strategy is validated. new_individual now draws min_population donors instead of recomputing three-or-five, so the count a candidate needs and the count the population is floored at are the same value by construction and cannot drift apart. Both subclasses clamp up to min_population with the same clamp-and-warn treatment the old floor of three already had, and the warning now names de_strategy and the required size.

This is the issue's first suggested fix, clamp and warn, chosen over raising so that a too-small population behaves for a 2 strategy exactly as it does for a 1 strategy today.

Tests

• The two population-floor tests are parametrized over all six strategies, pinning the floor at three for 1 and five for 2, single- and multi-island.
• Two regression tests drive a below-five 2 population through got_result -- the async path at population 4 and the island path at population 24 over 8 islands -- both of which raised before and now complete a generation.
• Verified against the pre-fix code: the six 1-strategy cases pass (controls); the eight 2-strategy cases fail.

docs/algorithms.rst notes the requirement where it already explains that the 2 strategies draw five parameter sets.