feat(genetic): make the fitness cache size configurable (#1376)

The per-run fitness cache of the GENETIC optimizer is unbounded. It
grows with individuals x generations (~110k keys at 300 x 400, about
1.1 kB each, 1.6 kB with an EV) and adds 120-170 MB to the peak memory
of a run, while only 6-15 % of the lookups hit. On 512 MB boards this
is the largest part of the optimizer's memory.

Add optimization.genetic.fitness_cache_max_entries:

- None (default): unbounded, the previous behaviour.
- 0: no cache; every genome is evaluated.
- N > 0: at most N keys. The cache is then an OrderedDict and evicts
  the oldest key first (FIFO, O(1)); LRU would cost bookkeeping on
  every hit for a cache that rarely hits.

A cache hit returns exactly what the deterministic evaluation would
return, so the plan for a fixed seed is identical with any setting.

Co-authored-by: Christin <christin@BigBrother.bikini.bottom.zone>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
Co-authored-by: Andreas <drbacke@gmx.de>

openapi.json

@@ -8801,6 +8801,24 @@
               42
             ]
           },
+          "fitness_cache_max_entries": {
+            "anyOf": [
+              {
+                "type": "integer",
+                "minimum": 0.0
+              },
+              {
+                "type": "null"
+              }
+            ],
+            "title": "Fitness Cache Max Entries",
+            "description": "Maximum number of keys in the per-run fitness cache. None = unbounded (default), 0 = cache disabled, N > 0 = keep at most N keys and evict the oldest first. Every cached evaluation takes one or two keys. The cache only saves repeated evaluations; the result for a fixed seed is the same with any value. Limit it to cap memory on small devices.",
+            "examples": [
+              null,
+              0,
+              5000
+            ]
+          },
           "measurement_max_age_seconds": {
             "type": "integer",
             "exclusiveMinimum": 0.0,
@@ -8949,6 +8967,24 @@
               42
             ]
           },
+          "fitness_cache_max_entries": {
+            "anyOf": [
+              {
+                "type": "integer",
+                "minimum": 0.0
+              },
+              {
+                "type": "null"
+              }
+            ],
+            "title": "Fitness Cache Max Entries",
+            "description": "Maximum number of keys in the per-run fitness cache. None = unbounded (default), 0 = cache disabled, N > 0 = keep at most N keys and evict the oldest first. Every cached evaluation takes one or two keys. The cache only saves repeated evaluations; the result for a fixed seed is the same with any value. Limit it to cap memory on small devices.",
+            "examples": [
+              null,
+              0,
+              5000
+            ]
+          },
           "measurement_max_age_seconds": {
             "type": "integer",
             "exclusiveMinimum": 0.0,