feat(inbox): walk the drop directory recursively

Door B listed `inbox.iterdir()` and kept only top-level files. A file in a
subdirectory was neither ingested nor refused: it appeared in none of the
result's buckets, so a nested drop produced a bundle that was silently short
of what was dropped and no count said so. That broke the K1b identity for any
inbox with folders in it. Operator decision 2026-09-06.

- `walk_inbox` is the ONE walk rule, shared with `tools/okf_corpus_run.py`:
  the denominator N is now counted over exactly the set of files the door
  ingests, rather than over a second listing that happened to agree.
- Sorted on the whole relative path, not the basename, so the order is a
  function of the tree; that is what keeps rebuild-from-scratch byte-equal to
  an incremental update.
- A concept's `source_file` is the path relative to the inbox root,
  `/`-separated. The concept NAME still comes from the basename, so two
  folders holding one basename hit the existing §3 collision refusal instead
  of one silently claiming the other's concept.
- Dot-directories and a bundle directory inside the inbox are skipped with a
  CODE, in a new `InboxResult.skipped`. Recursion makes the door's own output
  reachable as its own input; a silent skip would be the same
  absence-without-a-denominator defect one level down.
- `--path-prefix` reduces per component and rejoins with `/`, so the caller
  driving a nested corpus can carry the relative directory. Reducing the whole
  string folded the separator into a `-` and flattened `sub/sub2`.

`tests/test_inbox_flow.py::test_subdirectories_are_not_walked` asserted the
opposite and is superseded in place, with the reason written down.

Measured on the K2 corpus (flat, N=43): 39/43 merged, 4 coded, K1b holds. The
bundle digest is
`1472e98aec8643c5beee540f4c42b5e437bd26e7c61d69a91bcff799f06a6d13` over 1108
files -- byte-identical to a run of the same corpus at 190086f WITHOUT this
change (`diff -r` exit 0), so recursion costs a flat inbox nothing. It differs
from the stored 2026-09-03 artifact by one line in `index.md`
(`- [Corpus run history](log.md)`), which 95eb271 added 15 hours after that
bundle was built.

Suite 1113 passed, `ruff` clean, `mypy --strict src/ tools/` clean.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/test_propose_segments.py

@@ -304,6 +304,41 @@ def test_a_path_prefix_scopes_every_entry_under_one_directory(tmp_path: Path) ->
     assert parse_segmentation_plan(payload).entries
 
 
+def test_a_multi_component_prefix_carries_the_relative_directory(tmp_path: Path) -> None:
+    """Door B now walks the inbox recursively and records a `source_file`
+    relative to its root, so the caller driving a nested corpus has a
+    DIRECTORY, not a name, to scope by. Each component is reduced on its own
+    and rejoined with `/`: reducing the whole string would fold the separator
+    into a `-` and flatten `sub/sub2` into one component named `sub-sub2`,
+    which is a different bundle shape than the inbox it came from.
+    """
+    out = tmp_path / "plan.json"
+    assert (
+        okf_propose_segments.main(
+            [str(write(tmp_path)), "--out", str(out), "--path-prefix", "Bilag 3/Del II"]
+        )
+        == 0
+    )
+    payload = json.loads(out.read_text(encoding="utf-8"))
+    assert payload["entries"]
+    for entry in payload["entries"]:
+        assert entry["path"].startswith("bilag-3/del-ii/")
+    assert parse_segmentation_plan(payload).entries
+
+
+def test_a_prefix_with_one_empty_component_is_refused(tmp_path: Path) -> None:
+    """`a//b` and `a/###/b` are the multi-component form of the same defect the
+    single-component gate already refuses: an empty component would collapse
+    the path silently rather than scope it.
+    """
+    for prefix in ("sub//two", "sub/###/two", "/sub", "sub/"):
+        code = okf_propose_segments.main(
+            [str(write(tmp_path)), "--out", str(tmp_path / "p.json"), "--path-prefix", prefix]
+        )
+        assert code == 2, prefix
+        assert not (tmp_path / "p.json").exists(), prefix
+
+
 def test_without_a_prefix_the_artifact_is_byte_identical(tmp_path: Path) -> None:
     """Additive. Every plan already produced stays exactly what it was."""
     first = tmp_path / "a.json"