feat(lint): flag fui_ string resources that nothing in the module references

[demolaf]

• Related to chore(auth): delete 35 unreferenced fui_ string resources and their translations #2521

Adds an UnreferencedResource lint check that fails the build on a fui_* string or plurals resource :auth never reads.

Changes

• Collects declarations from the values/ folders, and references from R.string. and R.plurals. in Kotlin and Java and @string/ and @plurals/ in XML and the manifest.
• Excludes design-time tools: attributes, which is what found the last of the strings deleted in chore(auth): delete 35 unreferenced fui_ string resources and their translations #2521.
• Stacks on chore(auth): delete 35 unreferenced fui_ string resources and their translations #2521, so the check lands as a hard error with no baseline while the count is zero.

Why a custom check

UnusedResources is silent for a library module because lint cannot see its consumers, and checkDependencies against the demo app would flag nearly every resource.

The policy it encodes

No fui_* resource may be unreferenced within :auth itself. Normally the wrong rule for a library; it holds because the README documents these names as ones an app overrides rather than reads.

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Maintainer note: Fixes internal CPRN-488

[gemini-code-assist]

Code Review

This pull request introduces a new custom Android Lint detector, UnreferencedResourceDetector, designed to identify and flag unused fui_* string and plurals resources within library modules, along with its corresponding unit tests. Feedback on the implementation highlights a performance bottleneck in the XML scanning logic, where visiting every element and attribute and using element.textContent can lead to quadratic complexity. The reviewer suggests optimizing this by scanning XML files textually in beforeCheckFile and limiting element scanning to only string and plurals tags.