fix(pdfshxtext): .shx-only font resolution, oversized-object rule, mean text height

A listed font is compared only from its .shx file or the embedded stroke
font of that name; no TrueType or alias stand-in. Objects with a diagonal
over five times the selection's mean diagonal join no group. The created
MTEXT height is the mean of the character sizes. Text styles look a bare
font name up as .shx first, and a missing .shx name never draws with a
TrueType face. TXT2MTXT measures TEXT to its last inked stroke.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/scene/text/lff.rs

@@ -524,15 +524,32 @@ pub fn is_builtin(name: &str) -> bool {
 
 /// An alias names a real font (ISOCPEUR is a TrueType face): an installed
 /// copy of exactly that font draws it, Cyrillic included, where the LFF
-/// stand-in has no such glyphs (#1764).
+/// stand-in has no such glyphs (#1764). A name written as an `.shx` file
+/// never means the TrueType face: a missing `isocpeur.shx` or `swiss.shx`
+/// draws with the stroke substitute, not the installed `.ttf` (measured).
 fn installed_alias(stem: &str, name: &str) -> bool {
     use crate::scene::text::sysfont;
-    ALIASES.iter().any(|(alias, _)| *alias == stem)
+    !is_shx_name(name)
+        && ALIASES.iter().any(|(alias, _)| *alias == stem)
         && (sysfont::family_for_file(name).is_some()
             || sysfont::canonical_family_name(stem)
                 .is_some_and(|family| family.eq_ignore_ascii_case(stem)))
 }
 
+/// Whether `name` is written as an `.shx` file name.
+pub fn is_shx_name(name: &str) -> bool {
+    name.trim().to_ascii_lowercase().ends_with(".shx")
+}
+
+/// Whether an embedded font is that very font (its own stem, as `simplex`
+/// or `romans.shx`), not a stand-in reached through an alias (`txt` draws
+/// with `standard`).
+pub fn is_embedded_font(name: &str) -> bool {
+    let key = name.trim().to_ascii_lowercase();
+    let stem = key.strip_suffix(".shx").unwrap_or(&key);
+    FONTS_SRC.iter().any(|(s, _)| *s == stem)
+}
+
 pub fn get_font(name: &str) -> &'static Font {
     let map = fonts_map();
     let key = name.trim().to_ascii_uppercase();