feat(consume): measure the budget lock, add one flag-gated top-rank reservation

The prior measurement (docs/2026-09-08-blindsone-below-k-k2.md SS 3) found that
the budget, not the ranking, is the second lock on a mandate-shaped cost
question -- and that the same mechanism was a REGRESSION on the question that
works: raising `--k` to 16 evicted the gold concept, because the exact knapsack
maximises a SUM of fused scores and has no opinion about rank, so twenty small
excerpts out-value one that costs 56.5 % of the budget.

Measured here on the same 629-concept bundle, with the three known-positive
figures from `4c699fd` reproduced first:

- Corpus distribution, denominator 629: median excerpt 857 B, max 223 391 B,
  3 concepts over the limit alone.
- Candidate rule (b), a corpus-derived budget, is FALSIFIED by two numbers: two
  defensible derivations are 49x apart on the same corpus, the small one turns
  the gold concept into `over_budget_alone` (13 refusals against 2), the large
  one changes nothing at the default k. A budget is the consumer's constraint,
  not a property of the corpus; `--limit` already belongs to the caller.
- Built instead, behind `--reserve-top-rank` (default OFF): the top-ranked
  candidate gets its bytes before the pack runs, AFTER the `over_budget_alone`
  pre-exclusion and never before, and the payload declares `budget.reserved`.
- It fixes the eviction: k=16 and k=24 deliver the gold concept at rank 1,
  costing one and two excerpts, and 20.4 % / 27.3 % FEWER o200k tokens.
- It changes the delivered list in 2 of 24 measured combinations -- both of them
  that eviction. In the other 22 the list, its order and `spent` are identical.
- It does NOT close the mandate-shaped blind spot: that concept ranks 10, not 1.
  The one delivering command is `--cost-vocabulary --k 12 --limit 160000`
  (62 149 tokens against 58 401), and that is a consumer's decision.

11 new tests (RED first), 7 mutations 7 red with an unmutated negative control
green before and after; two of the seven survived the first test set and the
tests were strengthened. Default payload byte-identical, both goldens unchanged.

Report: docs/2026-09-08-blindsone-laas2-budsjett-k2.md
Suite 1279 green, mypy --strict clean over 28 files, ruff clean.

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

README.md

@@ -117,7 +117,8 @@ cuts them to a bounded set and emits one payload. It decides nothing about the
 question; the skill that reads the payload does the judgement. It calls no
 model, opens no socket, imports nothing outside the standard library and this
 package, and takes no clock: the same bundle bytes and the same
-`(question, k, limit, cost_vocabulary)` produce byte-identical output.
+`(question, k, limit, cost_vocabulary, reserve_top_rank)` produce
+byte-identical output.
 
 `--cost-vocabulary` is off by default and widens one question class: it lets a
 declared list of cost/price/quantity terms bridge a question and a document that
@@ -125,6 +126,17 @@ name money with different words. The gate is the question — one naming no such
 term gets byte-identical bytes either way — and what it does and does not close
 is measured in `docs/2026-09-08-blindsone-below-k-k2.md`.
 
+`--reserve-top-rank` is off by default and answers a different objection: the
+budget is packed by an exact knapsack, which maximises a SUM of scores and
+therefore has no opinion about rank, so a top-ranked excerpt costing a large
+share of the budget is out-summed by many small ones. Measured, that made `--k`
+a dial that could EVICT the concept a question was asked about. The flag gives
+rank one its bytes before the pack runs — after the `over_budget_alone`
+pre-exclusion, never before — and the payload then declares
+`budget.reserved`. On a 629-concept corpus it changed the delivered set in 2 of
+24 measured combinations, both of them that eviction:
+`docs/2026-09-08-blindsone-laas2-budsjett-k2.md`.
+
 It emits the § 8 shape — `contract`, `bundle` (`bundle_id` plus a
 `sha256-tree:` content identity), `budget` (unit, instrument, limit, spent and a
 validated known-positive), `denominators`, `excerpts` and `withheld` — and every