chore(release): prepare v0.7.0

[kalwalt]

Release preparation for v0.7.0 — the no_std / embedded milestone.

Bumps the version 0.6.1 → 0.7.0 (minor: new backward-compatible feature; default features and existing APIs unchanged) across Cargo.toml ([package] + [workspace.package]) and package.json, and adds the 0.7.0 CHANGELOG.md section.

Headline changes since v0.6.1:

• no_std / embedded support across core, imgproc, calib3d, and video (issues Feature request: no-std support so purecv can run on microcontrollers such a esp32 #82/[no_std] Phase 1 — make purecv-core compile under no_std + alloc #83/[no_std] Phase 2 — purecv-imgproc under no_std (parallel & simd disabled) #84/[no_std] Phase 3 (exploratory) — evaluate video/calib3d on embedded targets #85).
• OpenCV-style logging facade with cv_log_*! / cv_bail! / cv_err! (feat(core): add OpenCV-style logging facade (cv::utils::logging equivalent) #80).
• MSRV set to 1.88.

After this merges, the dev → main promotion PR will carry the release to main for tagging.

The CHANGELOG.md entry was hand-written to match cliff.toml's grouping; regenerate byte-perfect with git cliff if preferred (the release workflow generates the GitHub Release notes automatically).

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