feat(core): add validated Path network foundation

[Xuepoo]

Summary

• add renderer-independent Path graph schema with stable vertices and cubic segments
• add atomic create-path and set-path-network commands with selection and history integration
• add exact cubic bounds, adaptive compound fill/stroke hit testing, alignment/resize support, and PageHitIndex parity
• document topology limits and public geometry helpers with a minor changeset

Verification

• bun run verify
• Tests cover the documented behavior.
• No browser, DOM, Canvas, or VectoJS dependency entered Core.

173 tests and 1,395 assertions pass across 14 files. Prettier, Oxlint, TypeScript, CJS/ESM and declaration builds, package-export checks, and npm pack --dry-run all pass.

Documentation

• README, CHANGELOG, and relevant vectojs-docs task evidence are current.
• A Changeset is included when a published package version must change.