Cursor: add skip_id, expect_id, skip_fragment for zero-alloc scanning

lib/liquid/cursor.rb

@@ -53,6 +53,11 @@ module Liquid
       @ss.string = source
     end
 
+    # Extract a slice from the source (deferred allocation)
+    def slice(start, len)
+      @source.byteslice(start, len)
+    end
+
     # ── Whitespace ──────────────────────────────────────────────────
     # Skip spaces/tabs/newlines/cr, return count of newlines skipped
     def skip_ws
@@ -80,6 +85,39 @@ module Liquid
     end
 
     # ── Identifiers ─────────────────────────────────────────────────
+    # Skip an identifier without allocating a string. Returns length skipped, or 0.
+    def skip_id
+      start = @ss.pos
+      b = @ss.peek_byte
+      return 0 unless b && ((b >= 97 && b <= 122) || (b >= 65 && b <= 90) || b == USCORE)
+      @ss.scan_byte
+      while (b = @ss.peek_byte)
+        break unless (b >= 97 && b <= 122) || (b >= 65 && b <= 90) ||
+                     (b >= 48 && b <= 57) || b == USCORE || b == DASH
+        @ss.scan_byte
+      end
+      @ss.scan_byte if @ss.peek_byte == QMARK
+      @ss.pos - start
+    end
+
+    # Check if next id matches expected string, consume if so. No allocation.
+    def expect_id(expected)
+      start = @ss.pos
+      len = skip_id
+      if len == expected.bytesize
+        match = true
+        len.times do |i|
+          if @source.getbyte(start + i) != expected.getbyte(i)
+            match = false
+            break
+          end
+        end
+        return true if match
+      end
+      @ss.pos = start
+      false
+    end
+
     # Scan a single identifier: [a-zA-Z_][\w-]*\??
     # Returns the string or nil if not at an identifier
     def scan_id
@@ -188,6 +226,25 @@ module Liquid
       @source.byteslice(start, @ss.pos - start)
     end
 
+    # Skip a fragment without allocating. Returns length skipped, or 0.
+    def skip_fragment
+      b = @ss.peek_byte
+      return 0 unless b
+      start = @ss.pos
+      if b == QUOTE_S || b == QUOTE_D
+        quote = b
+        @ss.scan_byte
+        @ss.scan_byte while (b = @ss.peek_byte) && b != quote
+        @ss.scan_byte if @ss.peek_byte == quote
+      else
+        while (b = @ss.peek_byte)
+          break if b == SPACE || b == TAB || b == NL || b == CR || b == COMMA || b == PIPE
+          @ss.scan_byte
+        end
+      end
+      @ss.pos - start
+    end
+
     # Scan a "QuotedFragment" — a quoted string or non-whitespace/comma/pipe run
     def scan_fragment
       b = @ss.peek_byte