Consolidates duplication that owning the native ingester left against the still-
live gallery-dl path, and fixes a parity gap the duplication hid.
A1 — shared quarantine: extract file_validator.quarantine_file (move to
_quarantine/<slug>/<platform> + write the .quarantine.json provenance sidecar).
gallery_dl._validate_and_quarantine and patreon_downloader._validate_path both
call it. PARITY FIX: the native path now writes the provenance sidecar it
previously skipped — threads the media url through for source_url.
A2 — make_run_stats(**counts) factory in gallery_dl for the canonical run_stats
key set; gallery_dl._compute_run_stats and ingest_core both build through it so
the shape can't drift (gallery-dl path gains a benign dead_lettered_count=0).
A3 — one safe_ext in utils/paths.py; importer._safe_ext (thin wrapper, kept for
the Path call sites + memory pointer) and patreon_client both use it. Closes the
double-impl of the URL-encoded-basename ext gotcha.
A4 — promote gallery_dl._truncate_log/_extract_errors_warnings to module-level
truncate_log/extract_errors_warnings; download_service calls them directly
instead of reaching through self.gdl for native-result log shaping. The
staticmethods stay as thin delegators for existing callers/tests.
Behavior-preserving except the A1 sidecar parity fix. Test: native quarantine
writes a .quarantine.json (test_patreon_downloader).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
bvandeusen