fix: keep explicit fork sessions out of compression lineage

[ai-ag2026]

Summary

• Mark /api/session/branch sessions with session_source="fork".
• Keep explicit WebUI forks out of compression-lineage collapse even when their parent row is not currently present in the sidebar list.
• Add regression coverage for both the backend marker and the sidebar lineage guard.

Root cause

The sidebar lineage collapse uses parent_session_id as a fallback lineage key when no explicit compression lineage metadata is present. That is useful for compression continuations, but explicit WebUI forks also have parent_session_id. When forking from an already forked/filtered session and the immediate parent is not in the current list, the new fork can be treated like a lineage continuation of its parent/root instead of remaining an independently selectable fork row.

Related reconnaissance

• Searched existing PRs/issues for fork from here fork, fork session fork parent, fork from fork session, and session fork parent.
• No open duplicate found.

Test plan

• node --check static/sessions.js
• python3 -m py_compile api/routes.py
• uv run --with pytest --with pyyaml python -m pytest tests/test_465_session_branching.py tests/test_v050253_opus_followups.py tests/test_session_lineage_collapse.py -q -o addopts=
• git diff --check
• Added-line static scan: 0 findings

Review note

Independent delegate review was unavailable earlier due timeout, so this PR is intentionally narrow and manually reviewed against the scoped diff.