[P0] Replace line-oriented method signature parsing with a structured X++ method model

[jackyclever]

CLI/Core scope

Provide reliable method facts for CoC analysis, references, call reasoning, and scaffolding.

Migrated from Aether #5; this capability belongs in d365fo-cli/Core.

Parser/AST evaluation is tracked in d365fo-cli #2.

Current problem

Line-oriented signature extraction and modifier splitting can misread leading comments, multiline declarations/attributes, default expressions, modifier permutations, and other valid X++ shapes.

Acceptance criteria

• Define a structured method contract: visibility, modifiers, static/instance, return type, name, ordered parameters, parameter types/names/default expressions, relevant attributes, source span, and diagnostics.
• Parse leading doc/line/block comments and multiline declarations correctly.
• Handle commas/parentheses/brackets inside strings and default expressions.
• Cover display, edit, internal, modifier permutations, constructors, and delegates.
• Preserve raw source separately from parsed facts.
• Unsupported or malformed syntax returns a structured diagnostic instead of guessed facts.
• Add golden tests for valid, multiline, commented, defaulted, ambiguous, and malformed cases.
• Evaluate the Microsoft XLNT/BP parser path in [P2] Evaluate Microsoft XLNT/BP parser infrastructure for X++ AST coverage #2 before committing to a custom parser.