Add midplane z-slice to TestCase and thread through runner

[trisha-ant]

Stacked on #1.

Why

Max-intensity projections collapse depth: overlapping body segments fuse into one bright band, destroying the discriminative feature for fold stages — how many times the body folds back on itself. The fold-stage accuracy gap (1.5fold 55–65%, 2fold 58–77%) is the primary bottleneck. To let a perception variant count discrete body-segment cross-sections at one plane, the harness needs to expose a non-projected z-slice alongside the projection.

This is the harness half of the z-slice subagent work — no perception logic here, just plumbing.

What

• benchmark/testset.py
  • New _create_slice_image(volume, z=None, max_dim=800) — renders one XY slice (defaults to z=Z//2), uses the same crop as the three-view projection so framing matches, returns base64 JPEG.
  • TestCase gains a midplane_b64 field; iter_embryo populates it.
  • Collapsed the duplicated load-volumes-or-not branch into one path.
• run.py
  • _accepted_optional_kwargs(perceive_fn) introspects the perception function's signature once and passes midplane_b64 only if the function declares it (or **kwargs). All 15 existing variants get an empty extra-kwargs set, so they're called exactly as before — zero churn to existing variants.
• program.md × 2 — document the optional kwarg.

4 files, +69 / −9.

Verification

• _create_slice_image produces valid JPEG (ffd8ff); default is byte-identical to z=Z//2; negative/over-range z clamps to boundaries; Z=1 volumes and the resize-to-800px path handled.
• All 15 registered variants → _accepted_optional_kwargs(...) == set() (no breakage).
• iter_embryo populates midplane_b64 in both load_volumes={True,False} modes (mocked volume loader).
• End-to-end run_variant delivers the kwarg as JPEG b64 when declared, omits it cleanly (no TypeError) when not.
• python run.py --help exits 0; all modules import cleanly.
• Pyright: 22→24, the 2 new warnings are PIL_Image lazy-import instances inside _create_slice_image — same pattern as the 7 pre-existing instances in the file; no other deltas.

[trisha-ant]

Subsumed into #5 — all commits are in that branch and the retrospective there covers this variant.