Add seed_strategy option for Monte Carlo sampling

Sampling all parameters in one LHS call means adding, removing or
reordering a single parameter reshuffles every other parameter, which
makes design matrices hard to extend without invalidating existing runs.

The new 'seed_strategy' key in general_input selects between:

- 'joint' (default): existing behaviour, unchanged.
- 'independent': each uncorrelated parameter and each correlation group
  is keyed off distribution_seed, so editing one parameter leaves the
  others bit-identical. Correlation groups stay single sampling units,
  so requested correlations are induced as before.

Author: larsevj

src/semeio/fmudesign/_excel_to_dict.py

@@ -234,6 +234,7 @@ def parse_value(value: object) -> object:
         "repeats",
         "correlation_iterations",
         "distribution_seed",
+        "seed_strategy",
         "rms_seeds",
         "background",
     }
@@ -247,7 +248,13 @@ def parse_value(value: object) -> object:
         raise LookupError(msg)
 
     # Copy keys over if they exist
-    keys = ["designtype", "repeats", "correlation_iterations", "distribution_seed"]
+    keys = [
+        "designtype",
+        "repeats",
+        "correlation_iterations",
+        "distribution_seed",
+        "seed_strategy",
+    ]
     for key in keys:
         if key not in generalinput:
             continue

src/semeio/fmudesign/config_validation.py

@@ -4,9 +4,26 @@
 
 import copy
 import numbers
+from enum import StrEnum
 from typing import Any
 
 
+class SeedStrategy(StrEnum):
+    """How Monte Carlo samples are seeded.
+
+    JOINT:
+        All parameters are drawn in one Latin Hypercube Sampling call (the
+        default). Adding, removing or reordering a parameter reshuffles every
+        other parameter.
+    INDEPENDENT:
+        Each parameter, and each correlation group, is seeded separately from
+        the base seed, so changing one leaves the others bit-identical.
+    """
+
+    JOINT = "joint"
+    INDEPENDENT = "independent"
+
+
 def validate_configuration(
     config: dict[str, Any], verbosity: int = 0
 ) -> dict[str, Any]:
@@ -67,6 +84,22 @@ def validate_configuration(
             f"{key!r} must be a non-negative integer or None, got: {config[key]}"
         )
 
+    # 'seed_strategy' controls how Monte Carlo samples are seeded.
+    # See the SeedStrategy docstring for what each strategy means.
+    key = "seed_strategy"
+    value = config.get(key)
+    if isinstance(value, str):
+        value = value.strip().lower()
+    if value is None or value == "none":
+        value = SeedStrategy.JOINT
+    try:
+        config[key] = SeedStrategy(value)
+    except (ValueError, TypeError) as err:
+        raise ValueError(
+            f"{key!r} must be one of {[s.value for s in SeedStrategy]}, "
+            f"got: {config[key]}"
+        ) from err
+
     # 'seeds' here is 'rms_seeds' in the input. It can be either:
     # - 'default' => gives seed numbers 1000, 1001, 1002, ...
     # - 'None'    => seed number not added