docs: record named-session reattachment findings (#365)

[MisterTea]

Problem

See #365 for the original report.

Changes

Records findings about named sessions, control-mode attach support, and stale-client takeover behavior.

Resolution status

This is investigation material only. It does not add user documentation or change reattachment behavior, so it does not resolve #365.

Verification

Reviewed against current master. Run the repository-required formatting and unit-test commands after any follow-up code changes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.14%. Comparing base (db4f6f6) to head (9092ed7).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #824      +/-   ##
==========================================
- Coverage   79.26%   79.14%   -0.13%     
==========================================
  Files         118      118              
  Lines       12633    12633              
  Branches     8143     8143              
==========================================
- Hits        10014     9998      -16     
- Misses       1552     1565      +13     
- Partials     1067     1070       +3     

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[MisterTea]

Superseded by #793, which implements named-session reattachment, listing, last-heartbeat visibility, and session lifecycle behavior with real integration coverage. This placeholder findings-only PR does not add a second competing implementation.