CODAP2-286: Audit of unused DG.* strings in en-US.json

[kswenson]

Summary

• Adds docs/unused-dg-strings-audit.md documenting ~85 apparently unused DG.* string keys and 2 key mismatch bugs found during string ownership analysis for the V2/V3 shared POEditor project
• This is a documentation-only PR; no code changes — actual cleanup will be done under CODAP2-286
• Before any strings are actually deleted, manual verification that they're unused should be undertaken.

Jira

CODAP2-286

Test plan

• Verify the audit document is accurate and readable

🤖 Generated with Claude Code

[kswenson]

The caseTable.resizeColumn issue is a simple bug that can be fixed in code. The componentTitleChange issue is trickier, because even though the key in the code looks right and the key in POEditor looks wrong, the POEditor version is ostensibly the source-of-truth at this point, and localizers have likely added translations under that key, so we either need to keep the POEditor version of the key or provide a migration path.

[bfinzer]

👍🏻LGTM