fix #20078: nimpretty --indent applies to keepIndents regions (#25985)

## Summary
- nimpretty with non-default \--indent\ (e.g. 3 or 10) produced invalid
indentation in if/block/try expression regions because layouter kept the
original column when \keepIndents > 0\ and ignored \indWidth\.
- Rebase the column onto \indWidth\ using the relative offset from the
enclosing block baseline (\indentStack[^1]\).

## Root cause
\parser.nim\'s \
imprettyDontTouch\ template sets \keepIndents\ for if/block/try
expressions. layouter in the \keepIndents > 0\ branch used \ ok.indent\
(source column) directly as \indentLevel\ without scaling by \indWidth\,
so lines in these regions kept the original column and misaligned with
the rest of the file when \--indent\ differed from source indent width.

## Fix
\\\
im
em.indentLevel = em.indentStack.high * em.indWidth +
                 (tok.indent - em.indentStack[^1])
\\\

Keeps the relative offset from the enclosing block baseline but rebases
onto \indWidth\. At default \--indent:2\ the offset equals \indWidth\,
so output is unchanged (backwards compatible).

## Testing
- 12 custom cases x 3 indent values (2/3/10) = 36/36 pass
- nimpretty self-test suite 7/7 pass (no regression at default indent)
- 5 keepIndents scenarios (if/block/try expression continuation
alignment) that failed at indent:3/10 now pass

Fixes #20078.

compiler/layouter.nim

@@ -451,7 +451,13 @@ proc emitTok*(em: var Emitter; L: Lexer; tok: Token) =
   elif tok.indent >= 0:
     var newlineKind = ltCrucialNewline
     if em.keepIndents > 0:
-      em.indentLevel = tok.indent
+      # Apply the requested --indent width to "don't touch" regions (if/block
+      # expressions) too: keep the relative offset from the enclosing block
+      # baseline, but rebase it onto indWidth. Otherwise a non-default
+      # --indent would leave these lines at the original column and inject
+      # invalid indentation (see #20078).
+      em.indentLevel = em.indentStack.high * em.indWidth +
+                       (tok.indent - em.indentStack[^1])
     elif (em.lastTok in (splitters + oprSet) and
         tok.tokType notin (closedPars - {tkBracketDotRi})):
       if tok.tokType in openPars and tok.indent > em.indentStack[^1]: