th verify: increase forge spawnSync maxBuffer / avoid output overflow

[snissn]

Context

• Follow-up from PR Sepolia happy-path: th verify (Etherscan+Sourcify) + chain-config recording #5 (th verify improvements).

Problem

• th verify runs forge verify-contract via spawnSync and captures stdout/stderr.
• Node’s default spawnSync stdout/stderr buffer can be exceeded in edge cases, causing verification to fail even if forge would otherwise succeed.

Proposal

• Set an explicit maxBuffer when invoking spawnSync('forge', ...) (e.g. 10–20MB).
• Consider adding a lightweight guardrail: if output is truncated/overflowed, persist a message into manifest.extensions.verification[chainId] indicating output was truncated.

Acceptance criteria

• th verify does not fail due to buffer overflow for large forge output.
• Verification results are still written to manifest.extensions.verification[chainId] with actionable error context.