chore(ci): bump github actions to node24 runtime; add 0.4.0 changelog entry

[BetelGuese]

Summary

Two housekeeping items bundled:

1. Bumps GitHub Actions versions in the CI and release workflows to runtimes backed by Node.js 24. Node.js 20 was deprecated in GHA as of 2026-04; forced migration happens 2026-06-02, full removal 2026-09-16.
2. Adds a [0.4.0] entry to CHANGELOG.md for symmetry with the prior [0.3.0] paired-release entry.

Actions bumped

Action	File(s)	From	To
actions/checkout	.github/workflows/ci.yml (3x), .github/workflows/release.yml	v4 (34e1148)	v6.0.2 (de0fac2)
actions/setup-node	.github/workflows/ci.yml (3x), .github/workflows/release.yml	v4 (49933ea)	v6.4.0 (48b55a0)
softprops/action-gh-release	.github/workflows/release.yml	v2 (3bb1273)	v3.0.0 (b430933)

All pins preserve the # <semver> comment pattern and use full commit SHAs (SHA-pinned per supply-chain best practice). No with: blocks, inputs, or job config were altered. Project node-version: "20" intentionally left unchanged — this is about the action JS runtime, not the build toolchain Node.

Test plan

• CI runs on this PR must all pass (verifies the bumped actions still work).
• No runtime code changes — local npm test, npm run type-check, npm run build all green before commit.

CI + docs only — no package.json version bump, no tag, no npm publish.