fix(binding): resolve elements by semantics identifier and non-String keys

[charliewwdev]

Fixes problem 3 of #52.

Summary

_findElementByKey matched ValueKey<String> and nothing else, so two common shapes were unaddressable:

• Design-system fields. They wrap the inner editable in a private GlobalKey and expose only Semantics(identifier: ...). An agent had no handle on them — the report notes that only the "focus the field, then send an empty key" fallback worked.
• Non-String keys. ValueKey<int>, enum-valued keys, etc. never matched at all.

Approach

Candidates are collected in tiers and the first non-empty tier wins:

1. widget key (ValueKey of any type, compared by string form)
2. Semantics.identifier
3. Semantics.label

Ordering matters: a label is human-readable text that can easily collide with an unrelated widget's key, so a real key must beat a semantics annotation. Within a tier, the topmost-route preference added in #53 still applies.

Widening to ValueKey of any type is a superset of the old behaviour — ValueKey<String> resolves exactly as before.

enter_text needed no change: _findEditableText already descends from the matched element to the nearest EditableText, which is precisely what makes a matched Semantics wrapper usable. The new test pins that descent so it cannot regress.

Flutter version floor

Bumped environment.flutter to >=3.19.0 — verified via git tag --contains on flutter/flutter#138331 that 3.19.0 is the first release containing SemanticsProperties.identifier. The previous >=3.0.0 was already inaccurate: WidgetsBinding.rootElement needs 3.9.

Test plan

New test/element_lookup_test.dart (7 widget tests):

• ValueKey<String> still resolves (no regression)
• ValueKey<int>(42) resolves via '42'
• Semantics(identifier: 'email_field') wrapping a TextField resolves, and an EditableText is reachable below the match
• Semantics(label:) resolves
• A widget key beats a colliding semantics label
• Topmost route wins when the same key exists on a background and a pushed route
• Unknown key returns null
• Not vacuous: 3 of 7 fail against the old matching logic (int key, semantics identifier, semantics label); 7/7 pass with the fix
• flutter analyze lib/flutter_skill.dart test/element_lookup_test.dart — no issues

findElementByKeyForTesting is added as a @visibleForTesting wrapper so the tie-breaking rules can be pinned without exposing the finder publicly.

🤖 Generated with Claude Code

[charliewwdev]

Correction to the version claim in the description above: the floor is 3.27.0, not 3.19.0.

3.19 covers SemanticsProperties.identifier, but lib/flutter_skill.dart:4552 already uses Color.withValues, which landed in Flutter 3.27. Verified by grepping the flutter/flutter tags — absent in 3.24.0, present in 3.27.0. Pushed as a follow-up commit.

This also explains a pre-existing failure worth flagging separately: dart analyze has been red on main, because .github/workflows/ci.yml pins FLUTTER_VERSION: 3.24.0 — below what the source needs:

error - lib/flutter_skill.dart:4552:12 - The method 'withValues' isn't defined for the type 'Color'.
error - packaging/npm/dart/bin/server.dart:1:8 - Target of URI doesn't exist: 'package:flutter_skill_npm/src/cli/server.dart'.

The second error is fixed by #61. The first needs FLUTTER_VERSION bumped to 3.27.0 in ci.yml (and release.yml), which I could not push — the available token lacks the workflow scope.