Add README banner (0.4.0+1)

[imedboumalek]

What

Adds a project banner to the top of the README and republishes as 0.4.0+1 so it shows on pub.dev.

• README.md — banner image under the # haptify heading, with descriptive alt text.
• doc/haptify_banner.png (+ doc/haptify_banner.svg source) — 1920×1080 banner: the audio → analyze → .ahap / .haptic.json / _haptic.dart pipeline, in Flutter/Dart blue.
• pubspec.yaml + CHANGELOG.md — 0.4.0 → 0.4.0+1 (build-metadata bump; docs-only rebuild, not a patch).

Why

pub.dev renders the README of the latest published version and can't re-render an existing one, so a docs change needs a new version number. 0.4.0+1 is the minimal bump: pub ranks build suffixes, so it sorts above 0.4.0 and becomes latest, without consuming the 0.4.1 patch number.

Reviewer notes

• Banner ships inside the package (renders on pub.dev from package contents). dart pub publish --dry-run clean; archive 194 KB.
• Docs/asset-only — no library, CLI, or test changes.
• Merging to dev will not auto-publish (dev-release.yaml only tags -dev.N prereleases). Publish is the gated dev → main + v0.4.0+1 tag flow.