chore(release): prepare v0.4.1

[bmatar]

Summary

Patch release for installed-package ergonomics:

• diff-results --format json now emits structured JSON instead of console text; markdown and SARIF diff formats are honored too.
• tactual-mcp --http accepts both equals-style and space-separated --port / --host flags.
• Benchmark suites resolve packaged fixture files, and benchmark HTML fixtures now ship with the npm package so npx tactual benchmark works from a fresh install.

Verification

• npm run release:verify
• fresh tarball install smoke checks for diff output, MCP HTTP flags, and packaged benchmark fixtures