feat(video,calib3d): no_std support (Phase 3)

[kalwalt]

Concludes the exploratory Phase 3 (#85) with "port both" and implements it: video (optical flow) and calib3d (pose estimation, SVD) now compile under no_std + alloc.

Why port (the exploration result)

The audit found no hard blockers:

• std usage was minimal — video had none, calib3d had 4 trivial std:: paths (f64::consts, cmp::Ordering).
• calib3d RANSAC (solve_pnp_ransac, find_homography, find_fundamental_mat) uses its own self-contained Lcg PRNG (fixed-seed, pure arithmetic), not the std-gated thread-local RNG that forced hough_lines_p to stay std-only in Phase 2.
• No HashMap, Instant, thread_local, or println in either module.
• The empirical probe produced 137 errors, all mechanical — the exact Vec/vec!/to_string/Float/std::→core::/format! classes as Phase 2.

Memory: calib3d's SVD runs on tiny matrices (trivial); video's optical-flow pyramids are heap-heavy but feasible at low resolution — documented as the one caveat.

Changes

• Un-gate video + calib3d (module + prelude re-exports) in lib.rs.
• Mechanical std::→core::, alloc imports, and num_traits::Float where concrete f32/f64 math is used. optical_flow's SIMD-only vec! import is gated behind simd.
• crates/no-std-smoke now also exercises rodrigues (calib3d) and build_optical_flow_pyramid (video), so the bare-metal build covers all four no_std modules.
• CI no_std Build job renamed to cover core + imgproc + calib3d + video.
• README: new "no_std / embedded support" section with a per-module support matrix (satisfies [no_std] Phase 3 (exploratory) — evaluate video/calib3d on embedded targets #85's documentation acceptance criterion).

Verification

• --no-default-features builds clean on host and thumbv7em-none-eabihf.
• 308 lib + 40 doc tests pass with default features; clippy/fmt clean on no-default / default / simd+parallel.
• The no_std smoke crate builds for thumbv7em exercising all four modules; ESP32-S3 examples build against this tree.

After this, the whole core + imgproc + calib3d + video surface is no_std; only features2d remains std-gated.

Closes #85
Refs #82

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