fix(ci): run doc-graph wall on docs-only PRs — retire documentation_only_skip bypass (#7023)

[gunbai-bot]

Summary

Diagnosis (#7023): The doc-reachability witness was not misclassified — dag/test/claim/doc_reachability_witness_test.dag already declares ReadsLiveTree (since #6654). The escape was floor admission bypass: the documentation_only_skip shell shortcut in gunbc.ci_spec exited before claim_executor, so selection never ran and the never-predict-skip lane never engaged. Verified by execution: compile_clean_floor_skip_witness returns documentation_only_skip on a docs-only injected diff.

Fix (no hand selection edge): Remove the docs-only floor shortcut. Docs-only PRs now run the normal floor (SelectionApplied); import-closure skips non-live-tree witnesses cheaply; ReadsLiveTree rows (doc-graph wall, corpus-read host-fed lenses) always run via the existing fail-closed lane.

RED control: floor_skip_discovery_witness adds doc_reachability_runs_on_docs_only_diff and doc_reachability_never_predicted_unaffected_on_docs_only_diff — proves by execution that a docs-only diff RUNS the doc-graph witness (skipped=0).

Broader audit (report only — not fixed here)

Census of SubstrateInputsOnly discovery entries with host-read builtin signals: 3 rows (none are doc-reachability):

• dag/test/claim/typed_witness_invocation_test.dag (WitnessBin.Run)
• src/v2/test/claim/extdeps_external_authority/corpus/live_clean_tree_anchor_drop_red_test.dag
• src/v2/test/claim/long/live_anchored_modules_clean_test.dag

Durable fix for the class belongs to the module-identity-storage-binding lane (effect-reach census → derived ReadsLiveTree).

Falsifier backstop

The nightly falsifier runs the full floor cold (no documentation_only_skip shortcut). #7023's orphan was caught on the merged-with-main floor (#7035), not on PR CI — confirming the falsifier works but PR CI was the gap this PR closes. No falsifier prediction-gap finding (the bypass prevented selection from running at all; falsifier would not have diverged on a row that never entered the corpus).

Test plan

• ctrl-build -- cargo build -p v1-compiler --release --bin floor_skip_discovery_witness && ./target/release/floor_skip_discovery_witness — all tests green (including new doc-reachability docs-only controls)
• ci.yml updated to match gunbc.ci_spec emission (documentation_only_skip block removed)
• CI on this PR (flip to ready)