[no_std] Phase 1 — make `purecv-core` compile under `no_std + alloc`

[kalwalt]

Part of #82.

Goal

purecv-core builds with --no-default-features (no std) using only core + alloc.

Tasks

• Audit all std:: usages in purecv-core (grep -rn "std::" crates/purecv-core)
• Make std a genuine opt-in feature (std = [] in Cargo.toml, not implied by default elsewhere)
• Add #![cfg_attr(not(feature = "std"), no_std)] + extern crate alloc; in lib.rs
• Replace std::vec::Vec → alloc::vec::Vec (behind cfg or unconditionally if always available via alloc)
• Replace std::f64::consts → core::f64::consts
• Check for any panic/unwind assumptions (std::panic, catch_unwind) — none expected in core, but verify
• Add a no_std smoke-test crate/example that compiles for a thumbv7em-none-eabihf or similar bare target, to catch regressions without needing full ESP-IDF toolchain in CI
• Update CI: add a job building purecv-core with --no-default-features

Out of scope

Rayon/parallel, SIMD/pulp, imgproc/video/calib3d — handled in later phases.