Avoid recursive stack exhaustion in SVN candidate traversal

[lzl51230]

Related to #1113.

The SVN candidate walker recursively descends directories while retaining several path buffers per stack frame. A deeply nested working copy can exhaust the watcher thread stack even when the total path remains below platform limits.

Replace recursive traversal with a heap-backed worklist or otherwise bound stack usage. Add a deep-directory fixture that demonstrates traversal remains stable.

[DeusData]

Tracked as a 0.9.2-rc stability blocker for #1113/#1127. Recursive traversal of an attacker- or repository-controlled directory tree must not consume unbounded watcher-thread stack. Prefer a bounded heap worklist and test a deep tree without depending on a platform stack crash as the assertion.