refactor(org-refile): migrate to cj-cache helper

Phase 5 step 3 of utility-consolidation, second of two cache migrations.  Same shape as the agenda migration: drop four state vars, replace the build-with-cache function with a thin wrapper around `cj/cache-value-or-rebuild', extract the slow scan into a pure-ish helper.

Add `cj/--org-refile-scan-targets' as the slow filesystem walk (org-roam node enumeration plus 30,000+ todo.org files across code-dir and projects-dir).  `cj/build-org-refile-targets' now reads as: detect background-build state, ask the cache helper for the value with the scan helper as BUILD-FN, route the original log lines through :on-hit / :on-build-success.

Drop four module-level state vars:
- `cj/org-refile-targets-cache'
- `cj/org-refile-targets-cache-time'
- `cj/org-refile-targets-cache-ttl'
- `cj/org-refile-targets-building'

Rewrite the existing test file to test wrapper behavior at the contract level (stub the scan helper, verify wrapper outcomes).  8 tests parallel the agenda test set: first-call builds, second-call uses cache, force-rebuild bypass, TTL expiration, empty scan, building-flag cleanup on success and error, and error propagation.

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