[onamfc]

Builds on #16 (merged).

Summary

Behavior change. onDeferredDeepLink was called on every launch with the attribution recorded at install, as documented, so an app whose callback navigated had to keep its own "already handled" flag. The iOS, Android, React Native and Expo SDKs call it once, on the launch that reports the install. Flutter now does the same:

• Launch that reports the install: the callback receives the install's link, or null for an organic install, as before. A callback registered after initialize() returns on that launch is still called immediately.
• Later launches: the callback isn't called. The attribution stays available from getInstallData().

Changes

• DeepLinkHandler.deliverDeferredDeepLink returns early when isNewInstall is false: no callback, no cached value to replay to a late-registered callback, no last-click open (unchanged).
• Docs: onDeferredDeepLink KDoc and the README's deferred deep link section.
• Version 0.4.0: minor, because it changes behavior. Apps on ^0.3.x keep 0.3.2 (fix(events): succeed on the event endpoint's reply and send each queued event once #16) until they move to ^0.4.0.

Tests

• a later launch delivers no deferred deep link: replaces a cached install does not replace last-click attribution, which asserted the per-launch delivery; it still checks last-click attribution isn't written, and that getInstallData() returns the cached link. Fails without the change.
• the launch that reports the install delivers the deferred deep link once.
• flutter test: 149 passed. flutter analyze and dart format: clean.