docs: prepare v0.3 release documentation

[CoderDeltaLAN]

Summary:

• Prepare v0.3.0 release documentation.
• Keep v0.3.0 documented as release-candidate state, not published.
• Keep package version at 0.2.3 in this docs-only phase.

Validation:

• 142 tests OK.
• ./scripts/check.sh OK.
• git diff --check OK.
• doctor, budget, and explain source-tree smoke checks OK.

Release gate:

• No tag, GitHub Release, or PyPI publication in this PR.
• Release requires report and audit first.