Add WasmSyntax IR and WAT text format writer

Add WasmGC backend infrastructure:
- src/wasm/syntax.sml: WasmSyntax module defining the Wasm binary AST
  including value types, heap types, GC instructions (struct/array/i31),
  reference instructions, exception handling (try_table), and module
  structure (types, funcs, imports, exports, data, elem, etc.)
- src/wasm/wat-writer.sml: WAT text format writer for debugging,
  outputting S-expression format with proper indentation and escaping
- Update Makefile and lunarml-common.mlb to include new sources
- Update .gitignore for test/wasm build artifacts
- Add test/wasm/ with factorial/fibonacci test (fact-fib.sml, Makefile,
  test-fact-fib.mjs) for future Wasm backend validation

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: minoki

.gitignore

@@ -8,6 +8,9 @@ test/*/*/*.lua
 test/*/*.mjs
 test/*/*/*.mjs
 test/**/*.output
+test/wasm/fact-fib
+test/wasm/fact-fib.wasm
+test/wasm/fact-fib.wat
 bin/lunarml
 bin/lunarml.debug
 thirdparty/smlnj-lib/

Makefile

@@ -70,6 +70,8 @@ sources = \
   src/cps/boxing.sml \
   src/backend.sml \
   src/nested.sml \
+  src/wasm/syntax.sml \
+  src/wasm/wat-writer.sml \
   src/lua/syntax.sml \
   src/lua/transform.sml \
   src/codegen-lua.sml \

src/lunarml-common.mlb

@@ -60,6 +60,8 @@ backend.sml
 nested.sml
 lua/transform.sml
 codegen-lua.sml
+wasm/syntax.sml
+wasm/wat-writer.sml
 js/syntax.sml
 js/transform.sml
 js/simplify-jumps.sml