Document IDO whitespace sensitivity and controlled comparison guidance

src/decompnet/consoles/n64/guides/ido.md

@@ -5,8 +5,8 @@ may need a different version (5.3 vs 7.1) or flags: tell the user, they can swit
 
 Traits
 - Unconditional jumps inside functions are `b label` (`beq $zero, $zero`).
-- IDO is sensitive to how code is written on a line. Splitting or joining statements
-  (`a = b; c = a;` vs `c = a = b;`) changes the output.
+- Expression structure can change output: `a = b; c = a;` and `c = a = b;` are
+  different expressions. Test this separately from changes to line breaks alone.
 - It reuses registers in a predictable order; extra temporaries often *don't* help, while reordering
   statements or changing where a variable is first assigned does.
 - Common idioms: `if (x) { } else { }` with empty blocks, `do { ... } while (0);` around a sequence,
@@ -23,3 +23,21 @@ Argument home slots
 Static functions
 - `static` functions may be called with a different convention only in the same file; for matching a
   single function it rarely matters, but keep the name the ROM uses.
+
+Source line layout (observed; mechanism unconfirmed)
+- Preserve a candidate's line breaks before trying automatic formatting or one-line storage.
+  A private IDO 5.3 same-source comparison matched with preserved whitespace (score 0),
+  while replacing every whitespace run with one space scored 120 and changed a call/load
+  ordering. This establishes layout sensitivity in that example, not a newline-only cause.
+  The broader pass added 22 matches, but header/global-declaration fixes also changed between
+  passes, so those 22 cannot all be attributed to formatting. Full-image acceptance is separate.
+- Source locations may influence compiler decisions, but instruction scheduling has not been
+  established as the cause here. Extraction, preprocessing and macro effects must be ruled out.
+- For a controlled check, keep the compiler executable, flags, headers, macros, symbol addresses
+  and linker setup fixed. Save both inputs; change only newlines between ordinary C tokens.
+  Do not move preprocessor directives, continue `//` comments, or change `__LINE__` uses.
+  Compare preprocessed token sequences and linked instruction bytes, not whole object files
+  whose debug records can differ. Repeat both builds to check determinism.
+- Keep formatting variants as separate attempts and rejudge after formatting matched C.
+  A per-function match must still pass the complete image/cartridge rebuild. Never change the
+  target bytes or judge to accommodate a formatting variant.