feat(materialize): sources takes a list and renders N flow mappings

PM decision B6 asked for a list-taking _render_sources so a concept can record
more than one source, and prescribed the block list as the emitted form. The
list is delivered; the block form is not.

Three measurements, not an argument. Our own parse_frontmatter skips indented
lines, so a block list round-trips to an empty value with every entry silently
gone -- and _is_ingest_owned reads through that same parser. The consumer B6
was written for accepts the multi-entry flow sequence and classifies a block
sequence as unreadable provenance, so block would hand it exactly the state it
cannot read. And B6's own acceptance test asks for a round trip through this
parser, which no block form can pass.

A single source renders byte-identically, so all six goldens are unmoved. The
unquotable-value gate now runs on every entry, not just the first. New code
sources_empty refuses an empty list.

1023 -> 1034 tests, including the negative control that pins the block form's
silent data loss.

src/llm_ingestion_okf/errors.py

@@ -129,6 +129,8 @@ class MaterializationError(IngestError):
       contains a character that would restructure the `sources` flow mapping
       (Door A, v0.2 profiles); refused rather than emitted, because the
       resulting document parses cleanly into a record no one wrote
+    - `sources_empty` — a `sources` list with no entries; `sources: []` reads
+      as a measured absence when it is the absence of a measurement
     - `inbox_slug_empty` — a dropped file's name reduces to an empty slug
       under the id grammar (Door B; never an invented fallback name)
     - `inbox_slug_too_long` — the generated inbox filename would exceed the