feat(consume): give every excerpt the name and the address an answer must cite

The pre-pass delivered the right concept and the answer could not name it.
Measured by portfolio-optimiser 2026-09-08 over three paid arms: the gold
concept came back at rank 1 of 8 on 3 of 3 bundles, and the model answered
correctly on 1 of 3, because a delivered excerpt carried `concept_id`, body
text and nothing the document is known by. The previous session measured the
same gap from the other side: the provenance it had just written into every
concept did not reach the payload at all.

`excerpt_for` now carries `title` unconditionally, and `req_number`, the SPEC
5.1 address `sources` and each locator key (`source_pages`, `source_sheet`,
`source_rows`, `source_lines`, `source_offset`) when the concept has them. A key
the producer did not write stays absent: an empty value would assert that they
wrote an empty one, which is the contract's 6.4 failure.

`sources` is read in BOTH YAML forms, on a measurement rather than a taste. K2
writes the flow form on 629 of 629 concepts; the largest N-bundle writes the
block form on 270 of 270 and carries no locator key at all, so a flow-only
reader delivers that bundle with no address whatsoever. Reading the block form
is not a licence to write it - the emission rule is untouched, because the
line-oriented parser still cannot round-trip a block list. A `sources` value
this reader cannot decode is named (`sources_unreadable`), never dropped into
the same silence as an absent one.

Contract 8 gains the requirement and the checker gains its code
(`excerpt_unnamed`, 15 rules now, was 14): an excerpt a reader cannot name is
one an answer cannot cite, whatever its rank. `req_number`, `sources` and the
locators are SHOULD, not MUST - they are conditional on the producer, and a
bundle whose concepts carry no identifier cannot deliver one.

K2 controls, same question and same k, before against a frozen copy of the tool
at b6a8c8b: the RANKING does not move - the same 8 concept ids in the same
order, identical `text_sha256`, identical `withheld`, identical denominators
(629 = 621 + 8). The FIELD is what moved: payload 108 877 -> 111 744 B
(+2.63 %), budget spent 18 606 -> 20 907 (+287.6 B per excerpt), excerpt
members 9 -> 15, 83 changed lines. The contract document's own bytes moved with
8, so the budget instrument's known-positive moves with it: 10 349 -> 12 049
measured, 10 060 -> 11 719 raw, delta 289 -> 330.

New fixture `tests/fixtures/consume-provenance`: the two address forms and a
concept carrying neither address nor identifier. Purpose-built, because the two
real bundles are complementary and neither exercises both forms.

Suite 1347 (1339 before), ruff clean, mypy src clean.

Co-Authored-By: Claude <claude-opus-5>

docs/consumption-contract.md

@@ -157,7 +157,11 @@ are permitted and are not read by the checker.
       "concept_id": "<bundle-local id>",
       "sha256": "<hex digest of the concept file>",
       "adjudication": "proposed",
-      "trust_tier": "machine-confirmed"
+      "trust_tier": "machine-confirmed",
+      "title": "<the concept's own title>",
+      "req_number": "<the identifier the producer wrote, when there is one>",
+      "sources": [ { "resource": "<what the concept came from>" } ],
+      "source_pages": "<a locator into that resource, when the producer wrote one>"
     }
   ],
   "withheld": [ { "concept_id": "<bundle-local id>", "rule": "<why it was cut>" } ]
@@ -168,6 +172,25 @@ are permitted and are not read by the checker.
    equal `denominators.withheld`. The counts and the lists are two statements of
    the same fact, and a payload where they disagree is refused.
 2. `contract` MUST be present so a reader can tell which revision it is holding.
+3. Every excerpt MUST carry a non-empty `title`. An excerpt a reader cannot name
+   is one an answer cannot cite, whatever its rank. This is a measurement, not a
+   preference: in three paid arms run 2026-09-08 the pre-pass delivered the gold
+   concept at rank 1 of 8 on 3 of 3 bundles and the model answered correctly on
+   1 of 3, because the delivered unit carried `concept_id` and body text and
+   nothing the document is known by. § 3.1's tuple is an identity for a
+   *machine*; a title is what a citation is made of.
+4. An excerpt SHOULD carry the identifier the producer wrote (`req_number`), the
+   § 5.1 address of what it came from (`sources`), and the locator into that
+   address the producer wrote. These are SHOULD and not MUST because they are
+   conditional on the producer: a bundle whose concepts carry no identifier
+   cannot deliver one, and § 6.4 forbids reading their absence as a claim that
+   the source has none. A payload MUST NOT write an empty value in place of an
+   absent one, and MUST distinguish an address it could not read from one that
+   is not there.
+5. The locator keys are the *profile's*, not this contract's: SPEC § 5.1 has no
+   field for a place within a resource, so where a page number, a sheet row or a
+   line range is named — and under which key — is stated by the profile and read
+   from the concept, never invented here.
 
 ## 9. Prohibitions