feat: add native Octane diff view adapter

[zhsama]

Dependency

Blocked by MrWangJustToDo/reactivity-store#3. This PR must not be merged until that PR is merged and a reactivity-store version containing the reactivity-store/octane export is published.

The published dependency stays semver-based (reactivity-store: ^0.4.0); this branch does not pin or link to a personal fork.

Summary

• add @git-diff-view/octane with DiffView and DiffViewWithMultiSelect
• support unified/split rendering, themes, wrapping, highlighting, refs, widgets, and CSS exports
• ship separate browser and Node/SSR entries and bundled portable declarations
• add hook-slot isolation, multi-instance, remount, client, SSR, and packed-package tests

Validation

• Octane typecheck
• 6 client tests and 1 SSR test
• Octane client/server build
• packed browser/SSR consumer
• full package build and lint
• scoped Prettier check

Notes

The package cannot pass a clean registry-only install until the dependency above has been published. Local cross-repository validation used a temporary tarball only; no fork path is present in the committed manifest or lockfile.