Add v3.0.3 release notes to changelog

[kmcginnes]

Description

Add release notes for v3.0.3 covering all 37 PRs merged since v3.0.2.

Highlights:

• Schema Explorer — edge connections inferred from graph exploration (Infer edge connections from graph exploration #1742)
• Performance — faster schema operations for larger schemas (Rewrite schema merge for efficiency and referential equality #1735, Replace O(n²) lookups with Map-based atoms in schema state layer #1744)
• New feature — diagnostic logging toggle (Add diagnostic logging user setting #1709)
• Breaking change — cross-origin requests blocked by default (Block cross-origin requests by default #1738), with a dedicated section explaining the migration for non-standard deployments
• Infrastructure — hardened Docker image and CI, migrated to oxfmt and oxlint, TypeScript 6.0, proxy server runs natively

The release notes assume PR #1749 (AL2023 glibc bump) will be merged before the release.

Validation

• pnpm format passes
• Summary bullets are ordered by user-facing importance
• All 37 PRs listed in the All Changes section
• Permalink in the Cross-Origin Request Blocking section pinned to the v3.0.3 tag

Check List

• I confirm that my contribution is made under the terms of the Apache 2.0 license.
• I have verified pnpm checks passes with no errors.
• I have verified pnpm test passes with no failures.
• I have covered new added functionality with unit tests if necessary.
• I have updated documentation if necessary.