boot: survive hand-written sticks — case-fold the /system walk, skip host litter

The loader ENUMERATES /system now, so it sees whatever the stick's
directory entries literally store — and a hand-copied stick differs
from our generated image: firmware returns bare 8.3 short entries
UPPERCASE (INIT, SYSTEM), and host OSes leave litter next to every file
(macOS '._' AppleDouble forks, .fseventsd). Verified in QEMU/OVMF: an
uppercase-stored volume booted to a dead kernel-only system before this
change and boots fully (display up, logger writing /var/log) after.

The walk now lowers ASCII names (the danos tree is canonically
lowercase; FAT lookups are case-insensitive by definition), skips any
dot-prefixed entry, and treats malformed or unreadable entries as
skip-this-file instead of abort-the-whole-walk. Reader.find compares
case-insensitively as belt and braces.

system/initial-ramdisk.zig

@@ -76,17 +76,20 @@ pub const Reader = struct {
 
     /// Look a binary up by name: an exact path match wins; otherwise a unique
     /// basename match ("fat" finds "/system/services/fat") keeps pre-path callers
-    /// working. The returned Item's name is always the stored full path.
+    /// working. Comparisons are ASCII case-insensitive — the entries come from a
+    /// FAT volume, whose name lookups are case-insensitive by definition (and
+    /// whose short entries store uppercase). The returned Item's name is always
+    /// the stored full path.
     pub fn find(self: Reader, name: []const u8) ?Item {
         var i: u32 = 0;
         while (i < self.count) : (i += 1) {
             const item = self.entry(i) orelse continue;
-            if (std.mem.eql(u8, item.name, name)) return item;
+            if (std.ascii.eqlIgnoreCase(item.name, name)) return item;
         }
         i = 0;
         while (i < self.count) : (i += 1) {
             const item = self.entry(i) orelse continue;
-            if (std.mem.eql(u8, basename(item.name), name)) return item;
+            if (std.ascii.eqlIgnoreCase(basename(item.name), name)) return item;
         }
         return null;
     }