[no_std] Phase 3 (exploratory) — evaluate video/calib3d on embedded targets

[kalwalt]

Part of #82. Depends on Phase 2 (#).

Goal

Determine whether purecv-video (optical flow) and purecv-calib3d (pose estimation, SVD) should get full no_std support, or whether they should be excluded from embedded builds due to compute/memory cost.

Tasks

• Audit std:: usage in video/calib3d
• Rough compute/memory cost estimate for representative ops (e.g. one Lucas-Kanade iteration, one SVD) at typical embedded input sizes
• Decide: full no_std port vs. explicit "these modules require std" documentation
• If proceeding: repeat Phase 1/2 pattern (feature gating + no_std smoke build)
• If not proceeding: document limitation clearly in README/feature table so users know what to expect from an embedded build

Notes

This is explicitly lower priority — no commitment to full completion, this issue exists to make the scope decision explicit rather than let it be assumed.