[casoon]

Part of #778 (does not close it).

What

• audit_scope.requested_rules now lists every rule the run must account for with its viewports — shared, AX-tree (new engine::TREE_RULES table, the engine runs exactly it), page (PAGE_RULES executor and concrete ids) and inline rules (reflow, HTML content model; mobile only). The set is derived in wcag::expectation from the registrations, level and [rules] filter — no second list. audit_scope.wcag_level is new. Breaking vs. the unreleased 2.0 foundation: sample scope, coverage, session policy, snapshot fingerprint (#791 #778 #787 #790) #813 shape: requested_rules entries are {rule_id, viewports} objects, not strings.
• Validation: requested_rule_outcome (missing per viewport), rule_outcome_duplicate, rule_outcome_above_level.
• Counting unit: a page-rule executor and its concrete rules are one check (expectation::check_unit/count_checks) for failed_rule_checks, missing_capabilities and incomplete.
• Fixes found by a live Chrome run: context/on-input (static + live half) and the "not yet adopted" placeholders produced two outcomes per viewport; they now fold into one (WcagResults::fold_outcome, also used for frames). A requested html-conform check whose collection delivered nothing is an errored outcome, not a gap.
• Docs: schema, OUTPUT_CONTRACT, ARCHITECTURE, changelog.

Verified

cargo test (incl. rule_contract_gate, coverage_matrix, architecture ratchet), cargo clippy --all-features --all-targets -- -D warnings, cargo check --all-features; live --full audit of a local page passes validation (172 requested rules, 366 outcomes, no duplicates).

Still open in #778

• Frame pass: frames fold into page outcomes, but there is no per-frame outcome record (a frame whose rule errored only qualifies via the existing uninspected-frame path / folded errored status).
• Non-rule subchecks and State axes of the "per module, rule, subcheck, viewport and state" matrix (journeys, states from [2.0][U22] Interaktion und Sessionzustand sind kein abgeschlossener Messvertrag #787).
• Rule filter is not recorded in the scope, so a ran outcome for a filtered-out tree/page rule is not detected.
• Release CI/acceptance of the issue's remaining checkboxes.