feat(p18): an identifier that stands everywhere identifies nothing

P18 parts B and C (order 20260914T105139Z).

B1 -- stage 0b. P7 made it `item.code in grounding`: plain containment over
ONE concatenated string. P16 ran it against a delivered corpus and measured
what containment cannot tell apart: the falsification arm a4-indeksregulering
put 250 000 NOK on a single cost line coded R761 -- the knowledge base's OWN
NAME, carried by all 2 756 of its concept documents -- and the whole gate
said validated (stage 0 skipped, un-anchored run; checker approve).

The grounding is now carried as the DOCUMENTS it is made of (validator.
Grounding), not as a blob. A structure and not a second argument beside the
text: the boundaries and the text are one fact, and .text is derived, so the
gate and P8's report measure the same characters. run.py composes one
document per concept file where the base is already walked; generate.
_grounding_text folds each cost line in as a one-line document.

N and A are MEASURED, not chosen (14.09, four mounted vegnormal bases):
- every must_cite ref and mandate affected_code in the four context sets --
  shortest real identifier is FOUR characters (12.1, 52.1), so N = 3 sits one
  below the measurement and cannot refuse anything measured;
- document frequency of every code-shaped token per base -- 1 692 distinct
  and NOT ONE reaches 5 %. Highest anywhere 6/446 (1.35 %), highest a fasit
  names 3/446 (0.67 %), R761 2 756/2 756 (100 %). A = 0.05 therefore sits
  3.7x above the highest real token and 20x below the defect.
Length is NOT what makes the defect inert (R761 is four characters); the
share is. And a share is not a measurement without a denominator big enough
to take one (ansikt 4): one of three is 33 %, so an ABSOLUTE floor of 10
documents gates it. Highest absolute count any real identifier reaches is 6,
and every fixture in the repo is far below 10 -- which is why every pre-P18
gate is UNTOUCHED by this rule rather than exempted from it. Grounding.of
(one document) can never reach the floor by construction.

The refusal NAMES the denominator ("appears in 2756 of the 2756 documents
this run was given"), because Step 5 feeds that reason verbatim into the next
attempt's prompt: a proposer told only "ungrounded" answers with another
token of the same kind.

B2 SPIKE (measured, NOT built) FELLED the order's own alternative: option (b)
"ground in what the run OPENED" was run over P16's 16 code rows -- R761
stands in every OPENED document too, so (b) would NOT have caught the defect,
while B1 makes it inert and still grounds the real process line 65
ASFALTDEKKER (29/2756 = 1.05 %). (b) is not a substitute for B1.

C1 -- --docs-dir is optional once --bundle-dir is given (P16 FUNN 2). On the
bundle path docs_dir is never read: retrieval, the chunk tool and the "no
citable content" check all live in the road branch. Bound ONCE from
--bundle-dir, which is byte-identically what the README already tells an
operator to type by hand. NOT the "--docs-dir omvei": no such path is opened
and the road branch still refuses without a real --docs-dir (own arm).

C2 -- the judge's snippet arm counts only under citation_scope == "narrowed",
as (a) already does (PM decision, P16 s 6.2). P16's reason for (b') being
clean -- snippets are bodies while ref/title live in frontmatter, 0 of 446
n100 bodies -- holds for "Krav 4.1.2-1" but NOT for R761, where a process
number like 12.1 stands in the bodies. Under a whole-base citation list that
mark was "cited" before any model call.

tests: test_inert_identifier_loadbearing.py (7 arms; known positive is P16's
OWN artefact replayed against the base that run was given, known negative is
26 of 26 fasit references still grounding), test_docs_dir_optional_
loadbearing.py (5 arms). test_stress_judge_loadbearing.py's snippet arm split
into narrowed/whole-base -- the pair is the discriminator, same snippet, same
mark, only the scope differs. The grounding tests migrate from str to
Grounding.of (the honest reading of a caller that declared no boundaries).

Verification: uv run pytest -q 1698 passed / 5 skipped (1685 after part A,
strict superset, 0 removed). ruff check + format clean, mypy clean (38
files). Golden demo-transcript.stdout BYTE-UNCHANGED, shasum -a 1 of the
CONTENT = ea8c534773acdbe41ae68f2c55724d69aaf8be4f. No version bump, no push.

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

tests/test_grounding_offer_loadbearing.py

@@ -60,7 +60,7 @@ from portfolio_optimiser.reference_domain import CostItem, Project
 from portfolio_optimiser import run as run_mod
 from portfolio_optimiser.run import grounding_offer_notice, run_project
 from portfolio_optimiser.simulation import scripted_factory
-from portfolio_optimiser.validator import Rejection
+from portfolio_optimiser.validator import Grounding, Rejection
 
 FIXTURES = Path(__file__).parent / "fixtures" / "p7-grounding"
 SHARED = Path(__file__).resolve().parents[1] / "shared" / "examples"
@@ -125,7 +125,7 @@ def test_a_delivered_input_with_no_cost_line_reports_zero() -> None:
     measured form and the run is un-anchored, so the offer is null in BOTH numbers. Without this
     control a green (b) would be satisfied by a report that always counts positive."""
     for name in ("p6-k2-generation-prompt.txt", "s7c-k2-generation-prompt.txt"):
-        offer = grounding_offer(_project(), None, _prompt(name))
+        offer = grounding_offer(_project(), None, Grounding.of(_prompt(name)))
         assert offer.identifiers == 0, (name, offer)
         assert offer.cost_lines == 0, (name, offer)
         assert offer.chars > 0, "the measurement must have had text to measure"
@@ -140,7 +140,7 @@ def test_the_known_positive_input_reports_a_positive_offer() -> None:
     pass (a) and look measured."""
     text = _prompt("p4-n100-generation-prompt.txt")
     assert KNOWN_POSITIVE in text, "the fixture no longer carries the known positive"
-    offer = grounding_offer(_project(), None, text)
+    offer = grounding_offer(_project(), None, Grounding.of(text))
     assert offer.identifiers > 0, offer
     assert offer.cost_lines == 0, "a road standard carries no cost lines (F4's own finding)"
 
@@ -149,7 +149,7 @@ def test_a_bare_number_is_not_counted_as_an_offer() -> None:
     """LOAD-BEARING (b), the other half. K2 carries 46 394 bare-number occurrences over 2 117
     distinct values (P7 § 2), so counting them would make every report positive and the whole
     measurement inert — the repo's cardinal class, a gate that can only come out green."""
-    offer = grounding_offer(_project(), None, "1234 5678 90 42.5 1000000")
+    offer = grounding_offer(_project(), None, Grounding.of("1234 5678 90 42.5 1000000"))
     assert offer.identifiers == 0, offer
 
 
@@ -161,12 +161,12 @@ def test_the_offer_is_measured_on_the_text_the_gate_will_see() -> None:
     composer P7's gate uses, so the report and the gate cannot describe different texts. Both of
     the composer's OTHER two sources are exercised: a project cost line and a baseline code each
     raise the count, which a report built from ``delivered`` alone cannot do."""
-    delivered = "nothing citable here"
+    delivered = Grounding.of("nothing citable here")
     project, baseline = _project("PRJ-77"), _baseline("BAS-88")
 
     offer = grounding_offer(project, baseline, delivered)
 
-    assert offer.chars == len(_grounding_text(project, baseline, delivered))
+    assert offer.chars == len(_grounding_text(project, baseline, delivered).text)
     assert offer.identifiers == 2, offer
     assert offer.cost_lines == 1, offer