Add figure manifest to HANDOFF.md (Phase 6 of EPIC #290)

[jaredgalloway]

Closes #296

Adds a "Figure manifest" subsection to HANDOFF.md §5 mapping all 22
manuscript-included figures (via \includegraphics at f79ac4a) to
pipeline status: 10 regenerated (spike pipeline,
results-prod-294-naive-baseline-arm/), 12 missing — each with an
owner (#293, the new simulation-figures stub #316, or "out of
scope"). Also corrects a stale "S11–S15" reference to "S13–S15" at the
line documenting Phase 3's descope (S11/S12 are in fact regenerated).

Raised stub #316 for the simulation-figure gap (Figure 2, S1–S5) —
mostly renames of existing notebook outputs, not new analysis; full
breakdown in #296 and in the stub body.

Deviations from spec

None. Implementation matches issue #296 exactly: one HANDOFF.md
subsection, one stale-sentence fix, one stub raised. No pipeline,
Snakefile, notebook, or docs-page files touched; the derivation script
used to re-verify the manifest lives in _ignore/296-figure-audit/ and
is not committed.

Yolo trail: spec gate ✓, plan gate ✓ — full log in _ignore/yolo/296.log