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/ir.rs

@@ -84,6 +84,10 @@ pub enum IrOp {
     BeginUntil {
         body: Vec<IrOp>,
     },
+    /// BEGIN ... AGAIN (infinite loop)
+    BeginAgain {
+        body: Vec<IrOp>,
+    },
     /// BEGIN ... WHILE ... REPEAT
     BeginWhileRepeat {
         test: Vec<IrOp>,