[no_std] Phase 2 — `purecv-imgproc` under `no_std` (parallel & simd disabled)

[kalwalt]

Part of #82. Depends on Phase 1 (#).

Goal

purecv-imgproc builds under no_std + alloc with parallel and simd features off, falling back to scalar implementations.

Tasks

• Audit std:: usage in purecv-imgproc (canny, sobel, filters, morphology)
• Gate Rayon-based code paths strictly behind feature = "std" (Rayon requires OS threads, incompatible with no_std)
• Verify pulp (simd feature) no_std compatibility — check crates.io/docs
  • If not supported: gate simd behind feature = "std" too, for now
  • If supported: verify it actually builds for the bare target
• Confirm scalar fallback paths are used and correct when both parallel and simd are off
• Add CI job: purecv-imgproc --no-default-features on the same bare target as Phase 1
• Basic correctness check: run existing imgproc tests against scalar fallback to confirm no silent regressions from the feature-gating refactor

Out of scope

Actual ESP32/ESP-IDF integration testing (real hardware or QEMU) — could be a stretch goal but not required to close this issue. video/calib3d — Phase 3.