fix: Preserve Claude child parent references

[xingkaixin]

Claude child metadata was cached after discarding parent references whose transcripts had not appeared yet. Because the cache only tracked the child's metadata timestamp, a later parent file did not reconnect the child in a running instance.

Preserve the parent identity from Claude metadata or the transcript directory and let the session tree determine whether it can be mounted. Remove the duplicate parent-existence filtering and bump the head parser version so existing cached heads are rebuilt. Windowed scans still include true orphans and exclude children whose known parents are outside the selected window.

Validation:

• Reproduced a child remaining detached after its parent appeared in the same agent instance.
• Added coverage for delayed parents, retained parent references, windowed orphan selection, and old parser-cache invalidation.
• Passed local lint, formatting, typecheck, build, and tests.