prototype: add test-based corpus prefilling via AST analysis

[samalws-tob]

VIBE CODED PROTOTYPE. Still need to check whether this is correct, but it at least looks plausible
resolves #792

feat/prefill-corpus-verify branch contains instructions and example for verifying that this works

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Adds automatic corpus seeding by extracting call sequences from Foundry test functions. This helps bootstrap coverage-guided fuzzing with known-good transaction patterns from existing tests.

How it works:

• Parses contract AST to identify test contracts and test functions
• Extracts external function calls from test function bodies
• Converts calls to CallSequence format and adds to corpus
• Enabled via 'prefillCorpus: true' config option

Example:
contract TokenTest { Token token; function testTransfer() public {
token.mint(address(this), 1000); // Extracted
token.transfer(address(0x123), 100); // Extracted
}
}

This test function generates a 2-call sequence that seeds the corpus.

Implementation:

• Extended AST type system to parse function bodies (Block, FunctionCall, etc.)
• Added ast_analysis.go with test detection and call extraction logic
• Integrated into fuzzer initialization after corpus creation
• Uses existing solc AST data (no external dependencies)
• Case-insensitive contract name matching (variable 'counter' matches 'Counter')

Benefits:

• Bootstrap fuzzing with test-derived sequences
• Improve initial coverage by starting from known execution paths
• Pure Go implementation using existing compilation artifacts