Broken SVN external can freeze primary graph updates

[lzl51230]

Problem

The SVN observer currently produces one aggregate process/parser outcome. Any nonzero svn status exit or uncertain parse rejects the full observation. If one materialized external is broken, otherwise complete primary-working-copy edits cannot trigger graph synchronization until that external recovers.

This is a residual from the SVN auto-watch design in #1113.

Suggested direction

Track primary and external completeness/contributions separately. Preserve each incomplete external's last acknowledged contribution while allowing a complete primary observation to synchronize, and keep the external retry pending. Add real file/directory external fixtures with external-only failure.

[DeusData]

Thanks for separating this from the primary SVN observer work. I have classified it as a 0.9.2-rc follow-up task linked to #1113/#1127. Because #1127 currently claims a broken external cannot block primary synchronization, the PR review will treat this as a correctness blocker unless that claim and external participation are removed from the initial scope. The fixture needs independent primary and external completeness plus retry state.