Add random selection across mutation locations for ImplicitG/ImplicitF

[Copilot]

The conceptual mutator deterministically selected the first applicable mutation site, preventing generation of diverse mutants. For G((X a) -> (G b)), only ((X a) -> (G b)) was produced, never (G((X a) -> b)).

Changes

• New applyTilFirstRandom: Collects all applicable mutation paths via DFS, randomly selects one
• New collectAllMutationLocations: Returns list of paths ['operand'], ['left', 'operand'], etc. to mutation sites
• New applyMutationAtPath: Traverses path, applies mutation function, threads result back through parent nodes
• Selective application: Only ImplicitG and ImplicitF use random selection; complex mutations like BadStateIndex retain first-match behavior to preserve atomic transformations

Example

# Before: Always removes outer G
applyMisconception(parse("G((X a) -> (G b))"), ImplicitG)
# => "((X a) -> (G b))"  (100% of attempts)

# After: Randomly selects between removal sites
applyMisconception(parse("G((X a) -> (G b))"), ImplicitG)
# => "((X a) -> (G b))"  (~50%)
# => "(G((X a) -> b))"   (~50%)

Tested with formulas containing 2-3 mutation sites; distributions are approximately uniform. Existing tests pass without modification.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Conceptual mutator improvements</issue_title>
<issue_description>The conceptual mutator could be BETTER about generating more diverse candidates by running through more interesting ways of implementing the rewrite rules. For instance, for the formula G ( (X a) -> (G b)), we could potentially have two different implicit G mutants: ( (X a) -> (G b)) and G ( (X a) -> (b)). While its fine for the mutator to produce either one of them, I think we predictably always produce one of them every time.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Conceptual mutator improvements #104

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