fix(text): stroke aliases txt/monotxt must ignore a same-named TTF

Face::resolve routes a name to a system TrueType face when installed_alias
reports that an alias names a real TTF face (the #1764 escape hatch, added so
an installed ISOCPEUR draws that face instead of its LFF stand-in).

That hatch also caught TXT/MONOTXT: on a machine with a font family matching
"txt" (e.g. a "Txt Regular"), the DXF stroke text font resolved to that
TrueType face instead of the embedded stroke substitute, contradicting the
documented contract that a missing .shx draws with the stroke substitute and
never with a similarly-named TrueType face.

TXT/MONOTXT are stroke aliases, so exclude them from the hatch. ISOCPEUR keeps
its #1764 behaviour.

Eight tests failed on such a machine and now pass: the four font-face/LFF
assertions (shx_name_stays_lff, lff_falls_back_to_ttf_for_uncovered_glyph,
big_font_pairing_routes_ideographs_and_leaves_latin_alone,
lff_tessellation_golden) and the four searchable-PDF assertions that route
through the same Face::resolve. Adds a regression test for the contract.

src/scene/text/lff.rs

@@ -96,6 +96,13 @@ const ALIASES: &[(&str, &str)] = &[
     ("EXTFONT", "unicode"),
 ];
 
+/// Aliases that always mean the reference's *stroke* font and so never defer
+/// to an installed TrueType face of the same name: `txt`/`monotxt` are the DXF
+/// stroke text fonts, and a coincidental `Txt Regular` is not that font. The
+/// `installed_alias` escape hatch stays for aliases that genuinely name a
+/// TrueType face (ISOCPEUR, #1764).
+const STROKE_ALIASES: &[&str] = &["TXT", "MONOTXT"];
+
 /// The space advance of the reference's stroke fonts, in glyph units (cap
 /// height 9), measured from their text extents: the SHX names these LFF
 /// fonts stand in for keep the reference's word gaps.
@@ -529,11 +536,13 @@ pub fn is_builtin(name: &str) -> bool {
 /// draws with the stroke substitute, not the installed `.ttf` (measured).
 fn installed_alias(stem: &str, name: &str) -> bool {
     use crate::scene::text::sysfont;
+    let stem = stem.trim().to_ascii_uppercase();
     !is_shx_name(name)
+        && !STROKE_ALIASES.contains(&stem.as_str())
         && 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)))
+            || sysfont::canonical_family_name(&stem)
+                .is_some_and(|family| family.eq_ignore_ascii_case(&stem)))
 }
 
 /// Whether `name` is written as an `.shx` file name.