fix: preserve trailing comments on the same line as their statement

Comments like 'SELECT 2; -- trailing note' now stay on one line
instead of being split across two lines. The scanner detects when
a comment has no newline between it and the preceding token, marks
it as trailing, and the deparser appends it to the previous line.

Author: pyramation

packages/parse/__tests__/__snapshots__/roundtrip.test.ts.snap

@@ -22,8 +22,7 @@ exports[`fixture round-trip (CST) edge-cases deparsed output matches snapshot 1`
 SELECT 1;
 
 -- Inline comment after statement
-SELECT 2;
--- trailing note
+SELECT 2; -- trailing note
 
 -- Adjacent comments with no blank line
 -- first line

packages/parse/src/deparse.ts

@@ -44,7 +44,13 @@ export function deparseEnhanced(
 
   for (const stmt of result.stmts) {
     if (isRawComment(stmt)) {
-      lines.push(deparseComment(stmt.RawComment));
+      const commentText = deparseComment(stmt.RawComment);
+      if (stmt.RawComment.trailing && lines.length > 0) {
+        // Trailing comment: append to the previous line
+        lines[lines.length - 1] += ' ' + commentText;
+      } else {
+        lines.push(commentText);
+      }
     } else if (isRawWhitespace(stmt)) {
       // Each blank line in the original source becomes an empty line in output.
       // The whitespace node represents N blank lines between content.

packages/parse/src/parse.ts

@@ -85,6 +85,7 @@ function interleave(
             type: elem.value.type,
             text: elem.value.text,
             location: elem.value.start,
+            ...(elem.value.trailing ? { trailing: true } : {}),
           }
         }
       });

packages/parse/src/scanner.ts

@@ -21,6 +21,8 @@ export interface ScannedComment {
   start: number;
   /** Byte offset of the end of the comment (exclusive) */
   end: number;
+  /** True if this comment is on the same line as a preceding token (trailing comment) */
+  trailing: boolean;
 }
 
 export interface ScannedWhitespace {
@@ -87,13 +89,19 @@ export function scanComments(sql: string): ScannedElement[] {
     }
 
     if (token.tokenType === SQL_COMMENT) {
+      // A comment is "trailing" if no newline exists between the previous
+      // token's end and this comment's start (i.e. same line).
+      const gapBeforeComment = sql.substring(prevEnd, token.start);
+      const trailing = prevEnd > 0 && !gapBeforeComment.includes('\n');
+
       elements.push({
         kind: 'comment',
         value: {
           type: 'line',
           text: sql.substring(token.start + 2, token.end), // strip --
           start: token.start,
           end: token.end,
+          trailing,
         }
       });
     }

packages/parse/src/types.ts

@@ -12,6 +12,8 @@ export interface RawComment {
   text: string;
   /** Byte offset in the original source (for ordering) */
   location: number;
+  /** True if this comment is on the same line as the preceding statement */
+  trailing?: boolean;
 }
 
 /**