chore: prepare cortext.cpp v1.3.1 release

[gabewillen]

Prepares core v1.3.1 after the merged Node-API text-media bridge.

• Updates all core version surfaces and model manifest to 1.3.1.
• Documents that TypeScript-owned Node prebuilds are built from this exact core tag.
• No C++ behavior beyond the already-merged PR feat: expose text media through Node API #13.

Validated version consistency and diff checks.

[Copilot]

Pull request overview

Prepares the cortext core repository for the v1.3.1 release by updating all version-reporting “surfaces” and aligning the release artifact documentation with the already-merged Node text-media N-API bridge baseline (PR #13). This fits the repo’s release workflow by ensuring the engine, manifests, and build metadata all consistently report the new patch version without introducing new runtime behavior.

Changes:

• Bumps core version references from 1.3.0 to 1.3.1 across CMake, Zig build metadata, and the model manifest.
• Updates release documentation (RELEASE.md) to describe the v1.3.1 baseline (PR #13) and clarify Node prebuild provenance.
• Updates README asset download example to use 1.3.1.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
RELEASE.md	Updates v1.3.1 release artifact metadata and notes Node prebuilds are built from the matching core tag.
README.md	Updates the assets download example to use ASSETS_VERSION=1.3.1.
models/manifest.json	Bumps cortext_version in the model/assets manifest to 1.3.1.
CMakeLists.txt	Bumps the CMake project version to 1.3.1.
build.zig.zon	Bumps Zig package version to 1.3.1.
build.zig	Bumps Zig-reported version string and library semantic version to patch 1.

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