Add assert_cmd_snapshot function and Command class

[MatthewMckee4]

Summary

• Adds karva.Command builder class wrapping Rust's std::process::Command with .arg(), .args(), .env(), .envs(), .current_dir(), and .stdin() builder methods
• Adds karva.assert_cmd_snapshot() function for snapshot-testing command output (stdout, stderr, exit code) in insta-cmd style format
• Extracts inline snapshot call detection into find_snapshot_call() with extensible SNAPSHOT_CALL_PATTERNS constant
• Updates diff renderer to use box-drawing character (│) for separator column

Test plan

• 23 integration tests covering: basic echo, snap.new creation, nonzero exit codes, stderr capture, named snapshots, stdin, filters, env vars, inline matching/mismatch, builder chaining, multiple snapshots with allow_duplicates, current_dir, multiline output, empty output, stable re-run, and inline+name exclusivity
• Unit tests for format_cmd_output formatting
• Existing snapshot tests updated for new diff renderer characters

[codspeed-hq]

Merging this PR will not alter performance

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

✅ 1 untouched benchmark

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_{Comparing add-assert-cmd-snapshot (c6c0519) with main (febcd92)}