feat(inbox): fail-fast on filenames over the 255-byte limit

The slug inherited the whole filename stem with no length bound, so a long
dropped name produced a filename the filesystem cannot hold — surfacing as an
untyped OSError at the write, with an errno that differs per platform (63 on
macOS, 36 on Linux). The inbox contract promises a typed per-file outcome, so
the gate belongs in the pure function that forms the name.

Refusal, not truncation: truncating is lossy AND collision-prone — two long
names sharing a prefix would reduce to one filename and the second write would
silently claim the first file. Same posture as `inbox_slug_empty`: the library
never invents a filename the operator did not give it. The message carries the
actual size and the limit, since the fix is to rename the dropped file.

Limit verified empirically on APFS 2026-07-25: a 255-byte component writes, a
258-byte one raises errno 63. ext4 and NTFS cap at the same 255.

New MaterializationError code `inbox_slug_too_long`, registered in the errors
docstring and in the error-code conformance suite.

tests/test_error_codes.py

@@ -28,7 +28,7 @@ from llm_ingestion_okf.errors import (
     SourceError,
 )
 from llm_ingestion_okf.extract import extract_text
-from llm_ingestion_okf.inbox import inbox_slug, render_inbox_concept
+from llm_ingestion_okf.inbox import inbox_filename, inbox_slug, render_inbox_concept
 from llm_ingestion_okf.manifest import load_manifest, load_manifest_bytes
 from llm_ingestion_okf.materialize import materialize_bundle
 from llm_ingestion_okf.render import sql_value_to_text
@@ -363,6 +363,12 @@ def test_inbox_slug_empty() -> None:
     assert code_of(excinfo) == "inbox_slug_empty"
 
 
+def test_inbox_slug_too_long() -> None:
+    with pytest.raises(MaterializationError) as excinfo:
+        inbox_filename("a" * 247)
+    assert code_of(excinfo) == "inbox_slug_too_long"
+
+
 def test_inbox_title_invalid() -> None:
     with pytest.raises(MaterializationError) as excinfo:
         inbox_concept(title="broken [link]")

tests/test_inbox.py

@@ -140,6 +140,67 @@ def test_inbox_namespace_is_disjoint_from_the_reserved_names(name: str) -> None:
     assert generated.startswith("inbox-")
 
 
+# --- filename length: the one gate the filesystem enforces for us otherwise ---
+
+# NAME_MAX restated independently of the module constant, for the same reason
+# ID_GRAMMAR is: a change to the library's value must not silently move the
+# boundary this suite pins. Verified empirically on APFS 2026-07-25 (a 255-byte
+# component writes, a 258-byte one raises errno 63 ENAMETOOLONG); ext4 and NTFS
+# use the same 255 limit, and the slug is ASCII-only so bytes == characters.
+NAME_MAX = 255
+# `inbox-` (6) + `.md` (3) is the namespace's fixed overhead.
+LONGEST_ADMISSIBLE_SLUG = NAME_MAX - len("inbox-") - len(".md")
+
+
+def test_longest_admissible_slug_is_246_characters() -> None:
+    # Pins the arithmetic itself: if the namespace prefix or suffix ever
+    # changes, this fails before the length tests below start lying.
+    assert LONGEST_ADMISSIBLE_SLUG == 246
+
+
+def test_inbox_filename_at_the_length_limit_is_accepted() -> None:
+    slug = "a" * LONGEST_ADMISSIBLE_SLUG
+    generated = inbox_filename(slug)
+    assert len(generated.encode("utf-8")) == NAME_MAX
+
+
+def test_inbox_filename_over_the_length_limit_is_rejected() -> None:
+    """A name the filesystem cannot hold is a typed per-file failure, not an
+    untyped OSError from the write and not a truncation. Truncating would be
+    lossy AND collision-prone: two long names sharing a prefix would reduce to
+    one filename, and the second write would silently claim the first's file.
+    Refusing is the same posture as `inbox_slug_empty` — the library never
+    invents a filename the operator did not give it.
+    """
+    slug = "a" * (LONGEST_ADMISSIBLE_SLUG + 1)
+    with pytest.raises(MaterializationError) as excinfo:
+        inbox_filename(slug)
+    assert excinfo.value.code == "inbox_slug_too_long"
+
+
+def test_length_rejection_names_the_limit_and_the_actual_size() -> None:
+    # The operator's fix is to rename the dropped file, so the message has to
+    # say by how much — a bare "too long" leaves them guessing.
+    slug = "a" * 300
+    with pytest.raises(MaterializationError) as excinfo:
+        inbox_filename(slug)
+    message = str(excinfo.value)
+    assert str(NAME_MAX) in message
+    # 6 + 300 + 3 — the size the name WOULD have had.
+    assert "309" in message
+
+
+def test_a_long_dropped_filename_fails_at_the_filename_not_at_the_write() -> None:
+    """End-to-end on the pure functions: a 300-character dropped name reduces
+    to a 300-character slug, and the gate catches it before any I/O is
+    attempted — errno differs across platforms (63 on macOS, 36 on Linux), so
+    catching OSError by number was never a portable option.
+    """
+    with pytest.raises(MaterializationError) as excinfo:
+        inbox_filename(inbox_slug("x" * 300 + ".md"))
+    assert excinfo.value.code == "inbox_slug_too_long"
+
+
 # --- provenance rendering ---