T7.8 - B6: the wizard's actions ride the viewport, not the page

On the Constraints and Sequence steps the proposal's beside-the-fields actions
meant scrolling to save. The wizard's .step-navigation bar is now
position:sticky at the viewport bottom - the creator's sticky-bar pattern,
adapted rather than duplicated: sticky (not the creator's fixed) because the
bar lives inside the wizard's grid column, keeps its slot in the flow, and
therefore CANNOT obscure a field at any width - no padding arithmetic to get
wrong. Opaque background, top border and the shared --wp-shadow-sticky token
so content scrolling beneath it reads as beneath it.

The T4.4/B5 save-state indicator already mounted in this bar; it now rides the
viewport with the buttons, which is the "shows the save state" criterion.

Verification (each probe run alone): NEW tests/sticky_bar_check.py 12/12 - a
primary action inside the viewport on all 12 steps unscrolled at a 700px
viewport (short on purpose: both named steps genuinely overflow, asserted),
still visible fully scrolled, nothing obscured at 390px. Regression:
stepper_check ALL PASS.

Items: B6

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/reference/file-map.md

@@ -282,6 +282,7 @@ python tests/warning_check.py        # A2 - one warning, a badge from anywhere
 python tests/triage_check.py         # A6 - the sidebar answers the stand-up      16 checks
 python tests/qa_gate_check.py        # CR-014/D2/D9/D10 - QA gate + capture sink  40 checks
 python tests/files_check.py          # CR-007/D8 - drawings upload + real offline 36 checks
+python tests/sticky_bar_check.py     # B6 - save reachable on every wizard step   12 checks
 ```
 
 **Three probes were re-pointed at `T7.1`.** `sections_check.py` 5b drove the live