Define structured docs model and add unit tests

[RPG-Alex]

Summary

Define and expose a structured documentation model over SQL files (tables + columns + docs), and add tests to verify correct association of comments to AST elements.

Proposed data structures

struct SqlDocs {
    tables: Vec<TableDoc>,
}

struct TableDoc {
    name: String,
    doc: Option<String>,
    columns: Vec<ColumnDoc>,
}

struct ColumnDoc {
    name: String,
    doc: Option<String>,
}

Requirements

• Use PathBuf internally for file references to avoid lifetime issues (where needed).
• Ensure the model can be constructed from:
  • Parsed statements (sqlparser AST)
  • Extracted comment blocks from the original source text.

Tasks

• Implement SqlDocs, TableDoc, and ColumnDoc as the public-facing documentation model.
• Add constructors/builders to build SqlDocs from:
  • A parsed SQL AST + raw source text.
  • (Eventually) multiple files / a SqlFileSet.
• Add unit tests:
  • Comments directly above CREATE TABLE become TableDoc::doc.
  • Comments directly above a column definition become ColumnDoc::doc.
  • Inline comments on the same line as SQL are not treated as docs (unless you explicitly decide otherwise).
  • Interstitial comments (e.g. comments separated by blank lines or other SQL statements) are ignored as docs.
  • Multi-table fixtures where each table gets the correct comment block.