Harden API stability, tests, docs, and notices

[RockerFlower]

Summary

• Keep package version at 0.2.5; this PR does not prepare or publish a new release.
• Snapshot CustomConfig color, gradient, duration, and height lists so later caller mutations cannot change an existing config.
• Document legacy API surface: WaveWidget.heightPercentage is retained for compatibility and Layer remains exported only as a deprecated compatibility type.
• Replace the long inline Apache license header with SPDX plus NOTICE, and add LICENSE-APACHE-2.0 for the WaveView_flutter attribution path.
• Expand root widget/config tests and replace the empty example test with a real generator shell smoke test.
• Fix README install/docs links and refresh Cloudflare deployment docs to describe the current setup rather than the old migration state.

Verification

• fvm flutter analyze
• fvm flutter test
• fvm flutter test in example
• fvm dart pub publish --dry-run: 0 warnings