feat(import): Door C flow against an injected import gate (Phase 2 step 5)

Reads an external OKF bundle as {bundle-relative path -> document text},
hands it WHOLE to an injected gate over the guard's okf.import_bundle (a
bundle-level call: it resolves the cross-link graph across concepts), and
merges only concepts clearing the non-blocking floor. Same injection pattern
as Door B, so the core stays dependency-free while the CI channel for the
real guard is settled.

Two constraints shaped the design and are pinned by tests:

- A merged concept is written VERBATIM. Stamping provenance into it would
  require round-tripping its frontmatter through this library's line-oriented
  parser, which cannot represent the block lists the guard's parser accepts --
  silent data loss -- and would persist bytes the guard never screened.
- Ownership is therefore proven by content identity: identical bytes at the
  target name are a no-op re-merge (re-import of an unchanged bundle is
  idempotent), and anything else at the name is refused. Curated content and
  an updated concept are refused alike; refusing is what never destroys.

The floor is fail-closed beyond the plan's "no error" wording: an error, an
unrecognised disposition, and a concept the gate returned no verdict for are
all refusals. quarantine_review stays its own bucket, as at Door B.
origin/channel are validated against the guard's pinned vocabulary -- it
derives trust from origin by enum identity, so an unrecognised string would be
silently downgraded rather than caught.

Three primitives promoted for reuse rather than duplicated:
reduce_to_id_grammar and check_filename_length to materialize.py, and
extract.decode_text. Door C slugs the WHOLE concept path, so tables/users.md
and views/users.md stay distinct. Concept discovery folds case explicitly
rather than globbing *.md, whose case-sensitivity follows the filesystem and
would import the same bundle differently on APFS and ext4.

README's "what is gated today" section corrected: it claimed nothing is gated,
which is no longer true, but the honest statement is narrower than "the doors
are gated" -- the library cannot verify that an injected adapter is a real
guard, and a permissive stub is believed.

405 tests green; ruff, ruff format and mypy --strict clean.

src/llm_ingestion_okf/__init__.py

@@ -20,6 +20,14 @@ Door B (bundle inbox) public surface: process_inbox plus its result types.
 Its persist gate is INJECTED -- the caller supplies a Gate adapter over
 llm-ingestion-guard and process_inbox obeys the verdict; the library imports
 no guard function itself and makes no security decision of its own.
+
+Door C (external bundle import) public surface: import_bundle plus its result
+types. Its gate is injected the same way, over the guard's okf.import_bundle:
+the caller declares origin and channel at the door, the gate assesses every
+concept, and only concepts clearing the non-blocking floor are merged. Merged
+concepts are written verbatim -- ownership is proven by content identity
+rather than by a stamp, so an occupied name is only ever re-used when the
+bytes already there are identical.
 """
 
 from .errors import (
@@ -41,6 +49,16 @@ from .inbox import (
     PersistedFile,
     process_inbox,
 )
+from .importer import (
+    BundleDecision,
+    FailedConcept,
+    ImportDecision,
+    ImportGate,
+    ImportResult,
+    MergedConcept,
+    RefusedConcept,
+    import_bundle,
+)
 from .manifest import (
     Extraction,
     FileSource,
@@ -55,25 +73,33 @@ __version__ = "0.3.2"
 
 __all__ = [
     "BlockedFile",
+    "BundleDecision",
     "Extraction",
     "ExtractionError",
+    "FailedConcept",
     "FailedFile",
     "FileSource",
     "Gate",
     "GateDecision",
     "HttpSource",
+    "ImportDecision",
+    "ImportGate",
+    "ImportResult",
     "InboxResult",
     "IngestError",
     "IngestResult",
     "Manifest",
     "ManifestError",
     "MaterializationError",
+    "MergedConcept",
     "NetworkGateError",
     "PersistedFile",
+    "RefusedConcept",
     "RenderError",
     "SourceError",
     "SqlSource",
     "extract_text",
+    "import_bundle",
     "load_manifest",
     "materialize_bundle",
     "process_inbox",