Achieve 100% Core Extensions compliance, 261 tests

Implement 25+ Core Extension words:
- VALUE/TO, DEFER/IS/ACTION-OF, :NONAME
- CASE/OF/ENDOF/ENDCASE, ?DO, AGAIN
- PARSE, PARSE-NAME, S\", C", HOLDS, BUFFER:
- 2>R, 2R>, 2R@, U>, .R, U.R, PAD, ERASE, UNUSED
- REFILL, SOURCE-ID, MARKER (stub)

Fix panic on invalid memory access (bounds check in FIND).
Rewrite FIND/WORD host functions for inline operation.
Add BeginAgain IR variant and codegen.

Three word sets at 100%: Core, Core Extensions, Exception.

crates/core/src/dictionary.rs

@@ -193,6 +193,14 @@ impl Dictionary {
         self.latest
     }
 
+    /// Read the link field (previous word address) at a word entry.
+    pub fn read_link(&self, word_addr: u32) -> u32 {
+        if (word_addr + 4) as usize > self.memory.len() {
+            return 0;
+        }
+        self.read_u32_unchecked(word_addr)
+    }
+
     /// Allocate n bytes at HERE (like Forth's ALLOT).
     pub fn allot(&mut self, n: u32) -> WaferResult<u32> {
         let new_here = self