Never pin the scene to hardhat's implicit default solc

[shellygr]

Problem

On a hardhat project whose hardhat.config.js has no solidity entry (e.g. a brownie-generated network-only stub), npx hardhat config --json reports hardhat's built-in default compiler (0.7.3). HardhatManager took that as the project's compiler and pinned the entire scene to solc7.3 via compiler_map — on an all-^0.8.0 codebase every file (including our own injected DummyERC20Impl) fails with ParserError: Source file requires different compiler version, and compilation analysis dies before any workaround can help.

Two additional defects kept the existing recovery path from firing:

1. _detect_compiler_version_mismatch matched the ParserError phrase within a single line, but solc hard-wraps its diagnostics (same class as the Yul stack-too-deep wrap bug fixed earlier).
2. The compiler_version_mismatch workaround was registered with enabled=not solc_already_set, i.e. disabled exactly when a build system pinned a wrong global solc.

Fix

• hardhat_config_extractor.js additionally emits solidityImplicitDefault: hre.userConfig.solidity === undefined. When set, HardhatManager ignores the resolved compiler (logging why) and leaves solc_version=None, so compilation falls back to DEFAULT_SOLC_VERSION + per-contract pragma resolution.
• The mismatch detector matches the marker phrase across wrapped lines (\s+ between words), then maps the match back to a line index for the existing path/pragma context extraction.
• The workaround is enabled even when a global solc is configured: the detector only fires when that compiler provably cannot parse a file, and _seed_compile_maps promotes the scalar into compiler_map up front, so a per-contract override is always safe.

Validation

• 7 new unit tests (wrapped + single-line detection with a verbatim captured output, loop behavior with a wrong global solc, hardhat config extraction with implicit/explicit/legacy shapes); full suite passes (234).
• Extractor checked live in the mass-test container against a real brownie-stub project (solidityImplicitDefault: true, resolved 0.7.3 ignored) and a project with an explicit solidity block (false, 0.8.28 kept).
• End-to-end re-run of the failing project on this branch: compilation analysis now resolves the scene from pragmas (solc8.34) and completes solc compilation; the run proceeds to the next phase.

🤖 Generated with Claude Code

[jar-ben]

lgtm