feat: add --standard-json I/O mode to edgec

[refcell]

This implements issue #27. The edgec standard-json subcommand now implements the Foundry/solc standard JSON IPC protocol, making edgec a drop-in compiler backend for foundry-compilers.

The subcommand reads a JSON compilation request from stdin, compiles each source file listed in the sources map, and writes a JSON response to stdout. The response follows solc's standard JSON output schema with a top-level contracts map keyed by source path and contract name, each entry containing the ABI as a JSON array and the EVM bytecode as a raw hex string without a 0x prefix. All errors — parse failures, type errors, codegen failures — are encoded as structured objects in the errors array rather than signaled through the exit code, which always stays 0.

The implementation lives in a new crates/driver/src/standard_json.rs module that defines StandardJsonInput, StandardJsonOutput, and the compile_standard_json function. This function iterates over the input sources, calls Compiler::from_source for each, and assembles the output. As a side fix, the full bytecode path in compiler.rs was corrected to populate abi on CompileOutput — previously it was always None outside of --emit abi mode.

Four integration tests in crates/driver/tests/standard_json.rs cover successful compilation of counter.edge, structured error output on invalid source, JSON schema correctness, and the no-0x-prefix invariant on bytecode output. All workspace tests pass.

Closes #27.

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