feat(core): no_std support for the core module (Phase 1)

[kalwalt]

Makes the core module compile without the Rust standard library, using only core + alloc, so purecv can run on microcontrollers such as the ESP32 (#82).

std is now an explicit opt-in feature (still on by default — no change for existing users). With --no-default-features:

• lib.rs sets #![cfg_attr(not(feature = "std"), no_std)] with extern crate alloc
• all std:: paths in src/core moved to core::/alloc::; PureCvError implements core::error::Error (MSRV set to 1.88, which the codebase already required via is_multiple_of)
• float math (sqrt, sin, …, 243 call sites) resolves through num_traits::Float backed by libm, since those methods don't exist on f32/f64 without std
• genuinely std-only APIs are gated: get_tick_count/get_tick_frequency (no bare-metal clock) and the thread-local RNG (set_rng_seed, randu, randn, rand_shuffle)
• the not-yet-converted modules (imgproc, features2d, video, calib3d, features) and the parallel/simd/fft/ndarray features require std until their own phases land

A new build-only crate crates/no-std-smoke consumes the public API from a #![no_std] crate, and a new CI job builds both purecv and the smoke crate for thumbv7em-none-eabihf and lints the no-default-features configuration.

Verified: full test suite passes with default features (358 unit + 35 doc tests), clippy/fmt clean on both configurations, and matrix arithmetic ran end-to-end on real ESP32-S3 hardware (Xtensa, esp toolchain) with output logged over serial.

Alternative considered: splitting a purecv-core sub-crate for isolation — rejected for Phase 1 as the mechanical move would dominate the diff; the single-crate cfg_attr approach keeps the change reviewable.

Closes #83
Refs #82

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