docs: separate the 0.3.0 changelog entry from 0.4.0

[akriaueno]

Key Changes:

• Restore a dedicated ## [0.3.0] - 2026-06-08 section in CHANGELOG.md containing the spec-compliance work exactly as it shipped at the v0.3.0 release (its notes had been folded into the 0.4.0 section).
• Scope the ## [0.4.0] section to the changes that are actually new in 0.4.0: the pluggable authorizer redesign and per-request OAuth authorization-server / protected-resource-metadata resolution.
• Add the missing [0.4.0] and [0.3.0] comparison links at the bottom.

Testing:

• Documentation-only change; no code affected.

Other:

• Release housekeeping for v0.4.0. hex.pm already has 0.3.0 published, so the changelog now mirrors the published version history.

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• CHANGELOG.md

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