fix(runtime): add missing store instructions (0x3A-0x3E)

This commit adds support for partial-width store instructions that were
missing from the instruction parser, causing "Unknown instruction opcode"
errors when executing real-world WASM modules.

Added instructions:
- 0x3A: i32.store8  - Store 8-bit value from i32
- 0x3B: i32.store16 - Store 16-bit value from i32
- 0x3C: i64.store8  - Store 8-bit value from i64
- 0x3D: i64.store16 - Store 16-bit value from i64
- 0x3E: i64.store32 - Store 32-bit value from i64

These instructions are commonly used for:
- Byte-level memory manipulation
- String operations (UTF-8 encoding)
- Packed data structures
- WASI syscall buffers

Test results:
- ✓ simple_inference.wasm (WASI-NN) now executes successfully
- ✓ Fuel consumed: 60, Peak memory: 1202 bytes

The Instruction enum already had these variants defined; this commit
only adds the missing parser cases for the opcodes.

Impact: Unlocks execution of WASI modules and real-world applications

Author: avrabe

wrt-runtime/src/instruction_parser.rs

@@ -283,6 +283,61 @@ fn parse_instruction_with_provider(
                 memory_index: 0,
             })
         },
+        0x3A => {
+            // i32.store8
+            let (align, bytes1) = read_leb128_u32(bytecode, offset + 1)?;
+            let (offset, bytes2) = read_leb128_u32(bytecode, offset + 1 + bytes1)?;
+            consumed += bytes1 + bytes2;
+            Instruction::I32Store8(MemArg {
+                align_exponent: align,
+                offset,
+                memory_index: 0,
+            })
+        },
+        0x3B => {
+            // i32.store16
+            let (align, bytes1) = read_leb128_u32(bytecode, offset + 1)?;
+            let (offset, bytes2) = read_leb128_u32(bytecode, offset + 1 + bytes1)?;
+            consumed += bytes1 + bytes2;
+            Instruction::I32Store16(MemArg {
+                align_exponent: align,
+                offset,
+                memory_index: 0,
+            })
+        },
+        0x3C => {
+            // i64.store8
+            let (align, bytes1) = read_leb128_u32(bytecode, offset + 1)?;
+            let (offset, bytes2) = read_leb128_u32(bytecode, offset + 1 + bytes1)?;
+            consumed += bytes1 + bytes2;
+            Instruction::I64Store8(MemArg {
+                align_exponent: align,
+                offset,
+                memory_index: 0,
+            })
+        },
+        0x3D => {
+            // i64.store16
+            let (align, bytes1) = read_leb128_u32(bytecode, offset + 1)?;
+            let (offset, bytes2) = read_leb128_u32(bytecode, offset + 1 + bytes1)?;
+            consumed += bytes1 + bytes2;
+            Instruction::I64Store16(MemArg {
+                align_exponent: align,
+                offset,
+                memory_index: 0,
+            })
+        },
+        0x3E => {
+            // i64.store32
+            let (align, bytes1) = read_leb128_u32(bytecode, offset + 1)?;
+            let (offset, bytes2) = read_leb128_u32(bytecode, offset + 1 + bytes1)?;
+            consumed += bytes1 + bytes2;
+            Instruction::I64Store32(MemArg {
+                align_exponent: align,
+                offset,
+                memory_index: 0,
+            })
+        },
         0x3F => {
             consumed += 1; // Skip reserved byte
             Instruction::MemorySize(0)