Subject: Build fixes for GCC 15 (Fedora 43): avoid GCC 16-only libstdc++ APIs

Fedora 43 stays on GCC 15 for its whole life, while upstream targets GCC 16.
Two library additions used by Hyprland are missing from GCC 15's libstdc++,
so the compositor fails to build there. Fedora 44 (GCC 16) is unaffected.

1. std::ranges::starts_with (C++23, libstdc++ from GCC 16):
   "'starts_with' is not a member of 'std::ranges'". str_view is a lazy
   std::views::transform view, not a std::string, so the string_view member
   .starts_with() is not available. Use std::ranges::mismatch (C++20): the
   second range is fully consumed iff str_view starts with it.

2. std::basic_string::subview (C++26, libstdc++ from GCC 16):
   "'std::string' has no member named 'subview'". Construct a
   std::string_view over the request and take substr(), which yields the same
   non-owning view.

Upstream considers the original valid C++23/26, so this is a downstream
compatibility shim. Drop this patch once Fedora 43 reaches EOL.

--- a/src/helpers/MiscFunctions.cpp	2026-11-01 04:00:22.277308760 +0200
+++ b/src/helpers/MiscFunctions.cpp	2026-11-01 04:00:22.288198675 +0200
@@ -474,7 +474,7 @@
     });
 
     return [&](auto&&... prefixes) -> bool {
-        return (... || std::ranges::starts_with(str_view, prefixes));
+        return (... || (std::ranges::mismatch(str_view, prefixes).in2 == std::ranges::end(prefixes)));
     }("true"sv, "yes"sv, "on"sv);
     // clang-format on
 }
--- a/src/ipc/s1/S1.cpp	2026-11-01 04:00:22.283123322 +0200
+++ b/src/ipc/s1/S1.cpp	2026-11-01 04:00:22.289399160 +0200
@@ -145,7 +145,7 @@
             if (i < request.size() && (request[i + 1] == '\\' || request[i + 1] == ';'))
                 ++i;
             else
-                LOG(Log::ERR, "Malformed socket1 request: invalid escape sequence {} at position {}, using it verbatim", request.subview(i, 2), i);
+                LOG(Log::ERR, "Malformed socket1 request: invalid escape sequence {} at position {}, using it verbatim", std::string_view{request}.substr(i, 2), i);
         }
         parsedCommand << request[i];
     }