fix(scripts): distinguish absent vs failed title fetches; gate resume cursor (#236)

[williamzujkowski]

Summary

fetchWithRateRetry returned the same null for a title that is legitimately absent at a release point (301/302/404/doc-not-found) and for a transient failure (5xx after retries, 429 exhaustion, network error after retries). downloadAndExtractXml logged both as the misleading info line "Title not available", processBatch skipped them identically, and processReleasePoint advanced lastCompletedReleasePoint unconditionally — so a network blip during a multi-hour import permanently dropped a title's update from the historical git record, and --resume never retried it. The main loop also advanced the cursor on a thrown release point.

Fix

• Discriminate outcomes: TitleFetch = { ok } | { absent } | { failed }. fetchWithRateRetry returns a FetchOutcome; downloadAndExtractXml returns TitleFetch. A present-but-corrupt archive or missing XML entry is failed (worth retrying/surfacing), not absent.
• Gate the cursor: processBatch counts failedTitles; processReleasePoint advances lastCompletedReleasePoint only when failedTitles === 0. On failure it still saves manifests, so --resume reprocesses the release point and re-imports only the failed title (already-imported titles are no-op deltas — cheap and idempotent).
• Surface it: the main loop no longer advances the cursor on a thrown release point, and exits non-zero when any release point had unrecovered failures, so silent gaps no longer look like a clean run.

Verification

Classification verified via tsx (404/302/doc-not-found → absent; 200 ZIP → response; persistent 5xx / network error → failed).

Note: scripts/ is not a pnpm workspace package and imports @civic-source/shared/types which aren't symlinked at the repo root, so it has no CI test/typecheck (the pre-existing delta-detector.test.ts is similarly local-only). Wiring scripts/ into CI is a worthwhile separate follow-up; kept out of scope here. CI green reflects that workspace packages are unaffected.

Closes #236