Squelette 3.20.1 — the test fixture starts from blank decision registers

A fix to the template's own test suite. Not a single line of the controller changes; a project that never bound anything sees no difference.

What was broken. A derived project that had bound its volume 1 of Human Decisions (decision bind, introduced in 3.20.0) could no longer run the template suite on itself: 144 of 181 tests failed on one and the same line, DECISION_VOLUMES_CONSISTENT — cannot read docs/governance/HUMAN_DECISIONS.md at the binding origin … : the comparison term is unavailable. The control was right to refuse. The cause was in the test fixture: every test works on a copy of the project with no Git history (a single commit), and docs/governance/ was copied as it was — bound volume included. The volume names the commit it was bound at, a commit of the project's own history that no fixture copy has. Found on a real project one week after its binding; reproduced without it, on a throwaway derived project built from the template, with the very same numbers.

What changes.

• Fixture (tests/test_template.py, reset_to_not_started_fixture): the test copy starts from blank decision registers — a living notebook with no recorded decision and no summary (the summary is removed with the controller's own strip_decisions_summary), and no bound volume (HUMAN_DECISIONS_VOLUME_*.md removed from the copy). Volume 1 of real projects is untouched.
• New test (B15, test_the_suite_still_runs_from_a_derived_project_that_bound_its_volume): from a derived project whose volume is bound, it replays make_copy + bootstrap-audit, make_normal_copy + audit, then one test of the suite in a subprocess. Red on 3.20.0, green on 3.20.1.
• Core manifest at 3.20.1; demo and transcript regenerated; changelog, roadmap and view updated; the full diagnostic is under provenance/maintenance/2026-09-27-diagnostic-3.20.1-fixture-volume-relie.md (in French).

Verified. Template: 182 tests green (Python 3.10 and 3.11), audit, bootstrap-audit, traceability and demo.py --check PASS. A derived project with a bound volume, rebuilt on 3.20.1: 182 tests, 172 passed, 10 skipped (template-only tests), 0 failures — against 144 failures on 3.20.0.