Release Cortext v1.3.0

[gabewillen]

Prepare Cortext v1.3.0 after merged PR #11 (text-with-media API and binding ownership split).

• Bump all engine version surfaces to 1.3.0.
• Refresh RELEASE.md with the merged baseline, highlights, verification, and asset packaging targets.
• Keep sibling binding package versions independent; update only the core shared-assets example.

Validation: version consistency/JSON checks, zig fmt --check, and git diff --check pass. PR #11 exact-head CI and sanitizer checks are green.

[Copilot]

Pull request overview

Prepares the cortext.cpp engine repository for the v1.3.0 release by bumping version identifiers across build/package surfaces and refreshing the tracked release artifact documentation to match the post–PR #11 baseline.

Changes:

• Bump engine version surfaces to 1.3.0 (CMake, Zig package metadata, model manifest).
• Refresh RELEASE.md to document v1.3.0 baseline commit, highlights, verification, and packaging targets.
• Update the README release-assets download example to reference v1.3.0.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
RELEASE.md	Updates the tracked release artifact to v1.3.0 (baseline, highlights, verification, packaging, version surfaces).
README.md	Updates the assets download snippet to use the v1.3.0 release tag.
models/manifest.json	Bumps cortext_version in the model manifest to 1.3.0.
CMakeLists.txt	Updates the CMake project version to 1.3.0.
build.zig.zon	Updates Zig package version to 1.3.0.
build.zig	Updates Zig build version string and library semantic version to 1.3.0.

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