add missing diff socket logic

[hxadogar]

diff --sockets was registered but not actually any compared logic.
now shows sockets added or removed.
it handles all socket types.

i have tested and verified manually.
and i will try to follow up with a dedicated pr for tests.

[adrianreber]

Please add the tests to this PR as a second commit.

[github-actions]

Test Results

89 tests  +3   89 ✅ +3   4s ⏱️ -1s
 1 suites ±0    0 💤 ±0 
 1 files   ±0    0 ❌ ±0 

Results for commit bb8f01e. ± Comparison against base commit 34f655f.

♻️ This comment has been updated with latest results.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 72.91667% with 26 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.98%. Comparing base (34f655f) to head (bb8f01e).

Files with missing lines	Patch %	Lines
cmd/diff.go	72.91%	24 Missing and 2 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #210   +/-   ##
=======================================
  Coverage   79.98%   79.98%           
=======================================
  Files          16       16           
  Lines        1759     1854   +95     
=======================================
+ Hits         1407     1483   +76     
- Misses        263      281   +18     
- Partials       89       90    +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[hxadogar]

of course, will add tests shortly.

for now here is my manual testing output:

➜ sudo ./checkpointctl diff socket-8080.tar socket-8081_8082.tar --sockets
╔════════════════════════════════════════════════════════════════╗
║ Checkpoint Diff                                                ║
╠════════════════════════════════════════════════════════════════╣
║ Container: socket-test                                         ║
║ Image:     localhost/socket-test:latest                        ║
║ ID:        2088c8c5543c036e2b706e2ca743cd41c86c81b03037a524... ║
╚════════════════════════════════════════════════════════════════╝

Checkpoint A:
  Created: 2026-03-26T16:53:05+05:00
  Size:    5551762 bytes

Checkpoint B:
  Created: 2026-03-26T16:53:05+05:00
  Size:    5551910 bytes

┌─ Memory Changes ─────────────────────────────────────────────┐
│ = No change
└──────────────────────────────────────────────────────────────┘
┌─ Process Changes ────────────────────────────────────────────┐
└──────────────────────────────────────────────────────────────┘
┌─ Socket Changes ─────────────────────────────────────────────┐
│ Added:
│   + PID 1     TCP     0.0.0.0:8081 -> 0.0.0.0:0
│   + PID 1     TCP     0.0.0.0:8082 -> 0.0.0.0:0
│ Removed:
│   - PID 1     TCP     0.0.0.0:8080 -> 0.0.0.0:0
└──────────────────────────────────────────────────────────────┘
Summary:
Checkpoint comparison for container socket-test
Sockets: +2 -1

[hxadogar]

i have added bats tests and i think its ready for review

[adrianreber]

Thanks. Also add a couple of unit tests, especially for the error cases which are not covered via bats tests.

[hxadogar]

i added the unit tests and try to covered the error cases too (not too much)

you can take another look when you get time