Refactor EntityBehavior SEEK/FLEE to use PathProvider strategy; refs #315

EntityBehavior no longer holds a direct DijkstraMap reference. A new
PathProvider interface has three concrete implementations:

- DijkstraProvider: steps along a (possibly inverted) DijkstraMap. SEEK
  descends a normal map toward roots; FLEE descends an inverted map away
  from threats.
- AStarProvider: follows a pre-computed AStarPath step-by-step.
- TargetProvider: takes a single (x, y) target and picks the Chebyshev
  neighbor closest to it each turn.

Entity.set_behavior() gains a pathfinder= kwarg accepting any of the above
(DijkstraMap, AStarPath, or (x, y) tuple). The old executeSeek/executeFlee
helpers collapse into a single executeProviderStep() that delegates to the
provider.

EntityBehavior.h forward-declares PathProvider so the header stays light.
EntityBehavior::reset() moves out of line to avoid pulling PathProvider
into the header.

New tests: tests/regression/issue_315_path_provider_test.py covers all three
providers driving SEEK, FLEE via inverted DijkstraMap, mid-run pathfinder
swap, and invalid-argument handling. grid_step_bench baseline refreshed
against the new provider dispatch path.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/benchmarks/baseline/phase5_2/grid_step_bench.json

@@ -2,8 +2,8 @@
   "grid": "100x100",
   "entities": 100,
   "rounds": 1000,
-  "total_sec": 0.07824495097156614,
-  "mean_round_ms": 0.07824495097156614,
-  "p95_round_ms": 0.1227830071002245,
-  "per_entity_step_us": 0.7824495097156614
+  "total_sec": 0.06852402002550662,
+  "mean_round_ms": 0.06852402002550662,
+  "p95_round_ms": 0.09302806574851274,
+  "per_entity_step_us": 0.6852402002550662
 }