Consume the schema-8 families instead of merely checking them out

[ivanopcode]

Closes the false green the schema-8 impact analysis measured, on the Python side.
Mirrors curator PR 37 (relux-works/curator#37) for this implementation.

The hole

The declared candidate suite was 432eb2ee, whose conformance/v1/manifest.json
is byte-identical to the released pin 0c81c1f8 (sha256:12e58b82… on both).
Two immutable revisions, one conformance suite: every candidate run so far
measured exactly the corpus the default protocol lanes already measure, and no
assertion could have noticed. Identity authentication proves you read the right
bytes; it never proves the bytes differ from the pin, or that anything read them.

Schema 8 then landed (#40) with its corpora unread: IN_SCOPE_SCHEMA_NAMES in
tests/test_protocol_conformance.py is rc.6-era, and that module is pinned to
the released manifest anyway. agent-skill-v8, csk-skill-v8,
install-marker-v4, vectors/module-roots.json and
vectors/script-host-execution-policy.json were all checked out and never opened.

What changes

Presence and consumption are two different claims, so they get two ledgers.

Presence — .github/ci/candidate-artifacts.tsv declares the root-relative
artefacts each consumer's unguarded reads require. The partition is derived from
the root, never from the lane: a root publishing the surface serves the
consumer, a root publishing none of it defers it and names what was absent
(this is what the released-pin lanes do). The candidate lane runs
candidate_consumption.py require and exports
CSK_REQUIRE_FULL_CANDIDATE_ROOT=1, so a partly published surface fails rather
than running quieter.

Consumption — .github/ci/candidate-cases.tsv names the 11 tests the
candidate lane must observe passing on every runner, enforced by
candidate_consumption.py gate against that run's own --junitxml stream. A row
covers every parameterization of its test.

New consumer — tests/test_schema8_candidate_conformance.py authenticates
every byte against the candidate manifest and then decides real behaviour:

• 132 agent-skill-v8 + csk-skill-v8 cases through skillspec.load_skill_spec
  on a materialized snapshot;
• 27 install-marker-v4 cases through install_marker.read_install_marker, with
  a byte-exact to_json() round-trip;
• 10 module-roots vectors through the real go_v1 seams, asserting the
  diagnostic and the phase (go-list vs go-build), plus the published
  evaluation_order;
• the whole script-worker family: section classification in both directions, the
  closed policy identity and interpreter set against the suite's own bytes, every
  opt-in case's enforced/declared-only classification, and a refusal reaching the
  single shim publication point.

audit_label_cases is classified as a declared gap with its owner, not as
unreachable: this manager emits no script-command-declared-only /
script-command-unfiltered-declared-network label. Every other worker-side
section is refused before reached, and
test_a_refusal_precedes_every_worker_surface is the assertion those rest on.

Candidate — .github/ci/candidate-suite.json now declares 6001dc3
(sha256:803918bf…b44403, tree sha256:d2c9d3df…ed2656, 1.0.0-rc.9). The
released pin is unchanged in value and still read back to reject impersonation.

Marker substitution — consuming install-marker-v4 settles the four cases
#40's review logged as inherited schema-7 semantics. They are reader bugs:
common.schema.json admits revision, tag and branch identically on both
pins, so branch was never a schema-8 addition, and Core 4.2 binds a
local-path substitution to effective identity kind operator-local-git, a
network-git substitution to network-git, and a structured revision to the
effective object format. install_marker now decides all three:
install-marker-v4 goes to 27/27 and install-marker-v3 from 23/27 to 27/27
against the released pin.

Evidence

Removing any of the five schema-8 families from the candidate root fails the
suite, in both removal modes:

family removed	files gone from the checkout	family unpublished from the manifest
schema-cases/agent-skill-v8	pytest exit 1, 66 failed	require exit 1, named
schema-cases/csk-skill-v8	pytest exit 1, 66 failed	require exit 1, named
schema-cases/install-marker-v4	pytest exit 1, 27 failed	require exit 1, named
vectors/module-roots.json	pytest exit 2, collection error	require exit 1, named
vectors/script-host-execution-policy.json	pytest exit 2, collection error	require exit 1, named

A run that stops executing a declared case is green to pytest and red to the gate:
--deselect ...::test_module_root_case exits 0 with 173 passed, and
candidate_consumption.py gate --platform linux exits 1 naming the missing case.

Reverting only src/csk/install_marker.py turns
tests/test_install_marker_substitution.py 7-red and the consumer 4-red on
exactly the four cases the review named.

Local gates, all exit 0: consumer 183 passed against the rc.9 root; gate green
for linux/darwin/windows; tests/test_candidate_consumption.py 28 passed;
tests/test_install_marker_substitution.py 16 passed; tests/test_ci_workflow.py
13 passed; ordinary suite 1858 passed / 63 skipped; full
tests/test_protocol_conformance.py against the released pin 1044 passed / 1
skipped; Go E2E native selection against the rc.9 candidate; python -m mypy
75 files; actionlint.