Improve verification diff UX (field summary, changed-fields-only, truncation/full mode)

[rsanheim]

Summary

Polish verification failure output to make mismatches faster to diagnose, especially for large stdout/stderr snapshots.

Problem

Current failures can be noisy for large outputs and make it harder to identify the highest-signal differences quickly.

Proposal

Improve VerificationError messaging and diff presentation:

1. Field-level summary first

   • e.g. stdout changed, stderr unchanged, status changed

2. Show diffs only for changed fields

   • skip unchanged fields entirely

3. Use compact unified diff with context limit

   • configurable context lines (default small, e.g. 3)

4. Truncate very large diffs with clear marker

   • include "truncated" indicator and hint for full output option

5. Optional full diff toggle

   • env var or config option to force complete diff output

Example Failure Shape

Backspin verification failed!
Record: fixtures/backspin/example.yml

Summary:
- stdout: changed
- stderr: unchanged
- status: changed (expected 0, actual 1)

[stdout diff]
@@ ... @@
- old line
+ new line
...
(diff truncated, set BACKSPIN_FULL_DIFF=1 for full output)

Acceptance Criteria

• changed fields are clearly enumerated before diff blocks
• unchanged fields are omitted from diff section
• large diffs are truncated deterministically with explicit notice
• docs include guidance for full diff mode