feat(kebab-app): p9-fb-23 task 7 — early-skip Unchanged path in ingest

Adds the per-asset incremental-ingest skip block to all three flows
(markdown / image / pdf). When `IngestOpts::force_reingest = false`
AND the asset's blake3 checksum + parser/chunker/embedding versions
all match the existing DB record, ingest emits
`AssetFinished { result: Unchanged }`, bumps `aggregate.unchanged`,
and skips parse / chunk / embed / vector upsert entirely.

Shared `try_skip_unchanged` helper performs the four checks; per-flow
callers supply the active parser_version + chunker_version + optional
embedding_version. `force_reingest = true` bypasses the skip path so
`incremental_ingest::force_reingest_bypasses_skip` still sees `Updated`.

Tests:
- new `incremental_ingest.rs` covers both paths.
- existing `ingest_idempotent_on_second_run` /
  `re_ingest_image_produces_*` / `re_ingest_identical_pdf_produces_*`
  updated to assert `Unchanged` on identical-bytes re-ingest (the
  pre-task behaviour was `Updated`).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/kebab-app/tests/ingest_lexical.rs

@@ -52,10 +52,15 @@ fn ingest_idempotent_on_second_run() {
 
     let r2 =
         kebab_app::ingest_with_config(env.config.clone(), env.scope(), false).unwrap();
-    // Same files re-ingested — labelled Updated, not duplicated.
+    // Same files re-ingested — p9-fb-23 task 7 introduced the early-skip
+    // path: when checksum + parser/chunker/embedding versions all match,
+    // the second run reports `Unchanged` rather than `Updated`. Pre-p9-fb-23
+    // returned `Updated` here. The `force_reingest=true` path still returns
+    // `Updated` and is exercised by `incremental_ingest.rs`.
     assert_eq!(r2.scanned, 3, "second scan: {r2:?}");
     assert_eq!(r2.new, 0, "second run new should be 0: {r2:?}");
-    assert_eq!(r2.updated, 3, "second run updated: {r2:?}");
+    assert_eq!(r2.updated, 0, "second run updated: {r2:?}");
+    assert_eq!(r2.unchanged, 3, "second run unchanged: {r2:?}");
 
     // list_docs still has 3 docs (no duplicates).
     let docs = kebab_app::list_docs_with_config(