let domains author their own pass-2 estimator as formulas, not Python

Pass 2 previously only knew one estimator: a hardcoded physics model that
matches dimensions literally named platform/actuator/energy_storage. Any
domain outside that shape (e.g. archery) got all-zero estimates and failed
every combo. Domains can now declare free variables and per-metric formulas
as data instead; a safe AST-based evaluator (engine/formula.py, no eval())
resolves declared entity properties via dep(key, constraint_type) and
generalizes the existing hand-nested mass-budget search into an N-variable
recursive optimizer. Fully additive -- the legacy platform/actuator/
energy_storage path is untouched and still runs unchanged for every domain
that declares no formulas.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/test_snapshot.py

@@ -7,7 +7,7 @@ import pytest
 from physcom.db.schema import init_db
 from physcom.db.repository import Repository
 from physcom.models.entity import Entity, Dependency
-from physcom.models.domain import Domain, DomainConstraint, MetricBound
+from physcom.models.domain import Domain, DomainConstraint, FreeVariable, MetricBound, MetricFormula
 from physcom.models.combination import Combination
 from physcom.snapshot import export_snapshot, import_snapshot
 
@@ -169,6 +169,46 @@ def test_import_with_combinations(seeded_repo, tmp_path):
     assert len(fresh_combos) == len(data["combinations"])
 
 
+def test_export_import_roundtrip_free_variables_and_formulas(repo, tmp_path):
+    domain = Domain(
+        name="archery_snapshot_test",
+        metric_bounds=[MetricBound("drawback_force", weight=1.0, norm_min=0, norm_max=100)],
+        free_variables=[
+            FreeVariable(
+                name="draw_weight_chosen",
+                floor_formula='dep("draw_weight", "range_min")',
+                ceiling_formula='dep("draw_weight", "range_max")',
+                sort_order=0,
+            ),
+        ],
+        metric_formulas=[
+            MetricFormula(metric_name="drawback_force", formula="draw_weight_chosen * 2"),
+        ],
+    )
+    repo.add_domain(domain)
+
+    data = export_snapshot(repo)
+    exported = next(d for d in data["domains"] if d["name"] == "archery_snapshot_test")
+    assert exported["free_variables"] == [{
+        "name": "draw_weight_chosen", "sort_order": 0,
+        "floor_formula": 'dep("draw_weight", "range_min")',
+        "ceiling_formula": 'dep("draw_weight", "range_max")',
+    }]
+    assert exported["metric_formulas"] == [
+        {"metric_name": "drawback_force", "formula": "draw_weight_chosen * 2"},
+    ]
+
+    conn = init_db(tmp_path / "fresh.db")
+    fresh = Repository(conn)
+    import_snapshot(fresh, data, clear=True)
+
+    loaded = fresh.get_domain("archery_snapshot_test")
+    assert len(loaded.free_variables) == 1
+    assert loaded.free_variables[0].floor_formula == 'dep("draw_weight", "range_min")'
+    assert len(loaded.metric_formulas) == 1
+    assert loaded.metric_formulas[0].formula == "draw_weight_chosen * 2"
+
+
 def test_import_merge_skips_existing_domain(repo):
     """Merge import skips domains that already exist."""
     domain = Domain(