Diagnostic: warn on undeclared variables

[msarson]

Summary

Add a diagnostic warning for variables used in a procedure body that cannot be resolved to any declaration in scope.

Motivation

Currently the extension has no way to catch typos or missing declarations at edit time. The compiler catches these, but early feedback in the editor would improve the development experience.

Scope of work

Clarion variable scope is multi-layered — resolution must check in order:

1. Local procedure variables (declared before \CODE)
2. Parameters (from the \PROCEDURE()\ signature)
3. \LOC:-prefixed parameter aliases
4. Module-level variables (declared before the first procedure)
5. Global variables (from other \MEMBER\ files)
6. Class fields (when inside a method implementation)
7. Queue/Group/File fields (accessible after \SET/loop)
8. Built-in Clarion identifiers (\RECORDS, \ERRORCODE, \SELF, \PARENT, etc.)

Cross-file resolution is required for globals and class fields, making this expensive and prone to false positives if any layer is incomplete.

Risks / considerations

• High false-positive risk — any missed declaration source produces spurious red squiggles and erodes trust in all diagnostics
• \OMIT/\COMPILE\ conditional blocks mean some declarations may be conditionally absent
• \LIKE, \DIM, embedded group fields, queue fields are all valid declaration sources
• Should be opt-in / off by default via a setting until coverage is solid
• \ScopeAnalyzer\ and \SymbolFinderService\ provide a starting point but do not cover all cases

Suggested approach

Start narrow: single-file procedures only, no cross-file dependencies, flag only obvious unresolved identifiers on the left-hand side of assignments. Expand scope incrementally.

[msarson]

v0.9.2 infrastructure that helps with this

Several pieces of work landed in v0.9.2 that reduce the cost of implementing this diagnostic:

StructureDeclarationIndexer (SDI)

Provides an O(1) type-name → file lookup across all .inc/.equ files in the project's search paths. Helps with scope layer 6 (class fields) — the diagnostic can quickly confirm whether an identifier is a known class type name without scanning files.

IncludeVerifier

Resolves the two-level MEMBER → parent compilation unit → include scan chain. This is the same logic needed for scope layer 5 (globals from other MEMBER files) — the proven path is already there.

TokenHelper.isPositionInString

The diagnostic provider will need the same guard already added to Hover/F12/Ctrl+F12: don't flag identifiers inside string literals. The helper is already in TokenHelper and can be reused directly.

Token tree already models structure fields

oken.isStructureField, 	oken.structureParent, and 	oken.structurePrefix cover **scope layer 7** (Queue/Group/File prefix fields) without any additional scanning — DocumentStructure already builds this at tokenize time.

What's still the hard part

• Cross-file global variables (scope layer 5): the SDI only indexes type declarations (CLASS, QUEUE, etc.), not variable declarations (someVar LONG, THREAD). Those still require cross-file scanning.
• OMIT/COMPILE conditional blocks: genuinely hard — the tokenizer does not evaluate compile-time conditions, so some declarations may appear absent.

The suggested starting point in the issue (single-file, LHS of assignments only) is still the right approach — the v0.9.2 infrastructure makes the later expansion phases cheaper.