test: add InstanceConfig tests to identifier stability

Add tests for InstanceConfig to ensure instance identity behavior
remains stable across versions:

- instance_shared: Single instance used twice (backwards compatible)
- instance_separate: Two instances with same params (different IDs)
- instance_different_values: Different params (different IDs)

This ensures the InstanceConfig feature maintains its unique identifier
behavior for distinguishing between shared and separate instances.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: bpiwowar

src/experimaestro/tests/identifier_stability.json

@@ -16,6 +16,9 @@
   "float_negative": "67a86cea76bc90be4ec052e8c2f08829fb989e64a1d419541e68485dff85dba1",
   "float_simple": "f445ab15c80965e89436c9e58a369fb969f216f392e4cbb19846830731f9a1e4",
   "float_zero": "35879908ca1652ea07a4b49f0c44e9aa2ca264bedf14974dd9a765b1fafd1921",
+  "instance_different_values": "e175afb36163f56078beb516cf7489238b491571519e25c2b5ff68afbeccc643",
+  "instance_separate": "6d6274a5b541f60833e5d15a556a1b8adfaaa6dd0970a09a57849edd7a0c6fdd",
+  "instance_shared": "d9a76235da634b81b7559d561322263698852fa2012005781569154d7ad3cfc5",
   "int_negative": "4e2ad6ee44e1c9429b56900aea7ba81a6b746a38678d2e29a89805bfb32b9153",
   "int_positive": "2c57a590b8bf1bb5283a54d971814e196f395269f2973096dc277dbc24218962",
   "int_zero": "2696ea881e0f601d4ad75679560e0e3305fa2f15552952d88ac87df4cc6f9f49",

src/experimaestro/tests/test_identifier_stability.py

@@ -8,6 +8,7 @@
 from experimaestro import (
     Param,
     Config,
+    InstanceConfig,
     Task,
     LightweightTask,
     Option,
@@ -200,6 +201,24 @@ class CycleB(Config):
     a: Param["CycleA"]
 
 
+# --- InstanceConfig ---
+
+
+class SubModel(InstanceConfig):
+    """InstanceConfig for testing instance identity"""
+
+    __xpmid__ = "test.stability.SubModel"
+    value: Param[int] = 100
+
+
+class ModelContainer(Config):
+    """Config that contains SubModel instances"""
+
+    __xpmid__ = "test.stability.ModelContainer"
+    m1: Param[SubModel]
+    m2: Param[SubModel]
+
+
 def get_configurations():
     """Return all test configurations with their identifiers
 
@@ -288,6 +307,21 @@ def get_configurations():
     cycle_a.b = cycle_b
     configs["cycle_simple"] = cycle_a
 
+    # InstanceConfig - test instance identity
+    # Single instance used twice (shared) - backwards compatible with regular Config
+    sm_single = SubModel.C(value=100)
+    configs["instance_shared"] = ModelContainer.C(m1=sm_single, m2=sm_single)
+
+    # Two separate instances with same parameters - different identifiers
+    sm1 = SubModel.C(value=100)
+    sm2 = SubModel.C(value=100)
+    configs["instance_separate"] = ModelContainer.C(m1=sm1, m2=sm2)
+
+    # InstanceConfig with different parameter values
+    sm3 = SubModel.C(value=200)
+    sm4 = SubModel.C(value=300)
+    configs["instance_different_values"] = ModelContainer.C(m1=sm3, m2=sm4)
+
     return configs