fix: replace silent failures with loud errors in lookup tables and CLI

Lookup tables:
- Replace 16 placeholder F.zero() returns with @Panic for unimplemented
  MLE cases (UpperWord, Pow2W, VirtualROTR, VirtualXORROT*, etc.)
- Replace else => F.zero() / else => 0 fallbacks with panics that
  report the unsupported table index

CLI/commands:
- parseHexInput: report invalid hex bytes instead of silently treating
  them as 0
- run.zig: report emulator step errors instead of silent catch break
- run.zig: report register read errors instead of silent catch 0

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: MatteoMer

src/cli/args.zig

@@ -78,6 +78,7 @@ pub fn parseCommand(arg: []const u8) Command {
 }
 
 /// Parse hex string (with optional 0x prefix) into bytes.
+/// Returns null if allocation fails or the hex string contains invalid characters.
 pub fn parseHexInput(allocator: std.mem.Allocator, hex: []const u8) ?[]u8 {
     var clean_hex = hex;
     if (std.mem.startsWith(u8, hex, "0x") or std.mem.startsWith(u8, hex, "0X")) {
@@ -89,7 +90,11 @@ pub fn parseHexInput(allocator: std.mem.Allocator, hex: []const u8) ?[]u8 {
     while (i < buf_len) : (i += 1) {
         const start = i * 2;
         const end = @min(start + 2, clean_hex.len);
-        buf[i] = std.fmt.parseInt(u8, clean_hex[start..end], 16) catch 0;
+        buf[i] = std.fmt.parseInt(u8, clean_hex[start..end], 16) catch {
+            std.debug.print("Error: invalid hex byte '{s}' in --input-hex\n", .{clean_hex[start..end]});
+            allocator.free(buf);
+            return null;
+        };
     }
     return buf;
 }

src/commands/run.zig

@@ -46,7 +46,10 @@ pub fn runEmulator(allocator: std.mem.Allocator, elf_path: []const u8, show_regs
         std.debug.print("\n", .{});
         var running = true;
         while (running) {
-            running = emulator.step() catch break;
+            running = emulator.step() catch |err| {
+                std.debug.print("Emulator error during trace: {}\n", .{err});
+                break;
+            };
         }
 
         const max_steps = max_trace_steps orelse 100;
@@ -149,7 +152,10 @@ pub fn runEmulator(allocator: std.mem.Allocator, elf_path: []const u8, show_regs
             std.debug.print("\nFinal Register State:\n", .{});
             var reg_i: u8 = 0;
             while (reg_i < 32) : (reg_i += 1) {
-                const val = emulator.registers.read(reg_i) catch 0;
+                const val = emulator.registers.read(reg_i) catch |err| {
+                    std.debug.print("  x{d:0>2}: error reading register: {}\n", .{ reg_i, err });
+                    continue;
+                };
                 if (val != 0) {
                     const reg_name = switch (reg_i) {
                         0 => "zero",

src/zkvm/lookup_table/mod.zig

@@ -1361,7 +1361,7 @@ pub fn LookupTable(comptime F: type, comptime XLEN: comptime_int) type {
                 10 => SignedLessThan.evaluateMLE(r),
                 11 => UnsignedLessThan.evaluateMLE(r),
                 12 => Movsign.evaluateMLE(r),
-                13 => F.zero(), // UpperWord - TODO
+                13 => @panic("UpperWord MLE not implemented"),
                 14 => UnsignedLessThanEqual.evaluateMLE(r),
                 // 15 was ValidSignedRemainder (removed in PR #1355)
                 15 => ValidUnsignedRemainder.evaluateMLE(r),
@@ -1371,25 +1371,25 @@ pub fn LookupTable(comptime F: type, comptime XLEN: comptime_int) type {
                 19 => LowerHalfWord.evaluateMLE(r),
                 20 => SignExtendHalfWord.evaluateMLE(r),
                 21 => Pow2.evaluateMLE(r),
-                22 => F.zero(), // Pow2W - TODO
+                22 => @panic("Pow2W MLE not implemented"),
                 23 => ShiftRightBitmask.evaluateMLE(r),
-                24 => F.zero(), // VirtualRev8W - TODO
+                24 => @panic("VirtualRev8W MLE not implemented"),
                 25 => VirtualSRL.evaluateMLE(r),
                 26 => VirtualSRA.evaluateMLE(r),
-                27 => F.zero(), // VirtualROTR - TODO
-                28 => F.zero(), // VirtualROTRW - TODO
-                29 => F.zero(), // VirtualChangeDivisor - TODO
-                30 => F.zero(), // VirtualChangeDivisorW - TODO
-                31 => F.zero(), // MulUNoOverflow - TODO
-                32 => F.zero(), // VirtualXORROT32 - TODO
-                33 => F.zero(), // VirtualXORROT24 - TODO
-                34 => F.zero(), // VirtualXORROT16 - TODO
-                35 => F.zero(), // VirtualXORROT63 - TODO
-                36 => F.zero(), // VirtualXORROTW16 - TODO
-                37 => F.zero(), // VirtualXORROTW12 - TODO
-                38 => F.zero(), // VirtualXORROTW8 - TODO
-                39 => F.zero(), // VirtualXORROTW7 - TODO
-                else => F.zero(),
+                27 => @panic("VirtualROTR MLE not implemented"),
+                28 => @panic("VirtualROTRW MLE not implemented"),
+                29 => @panic("VirtualChangeDivisor MLE not implemented"),
+                30 => @panic("VirtualChangeDivisorW MLE not implemented"),
+                31 => @panic("MulUNoOverflow MLE not implemented"),
+                32 => @panic("VirtualXORROT32 MLE not implemented"),
+                33 => @panic("VirtualXORROT24 MLE not implemented"),
+                34 => @panic("VirtualXORROT16 MLE not implemented"),
+                35 => @panic("VirtualXORROT63 MLE not implemented"),
+                36 => @panic("VirtualXORROTW16 MLE not implemented"),
+                37 => @panic("VirtualXORROTW12 MLE not implemented"),
+                38 => @panic("VirtualXORROTW8 MLE not implemented"),
+                39 => @panic("VirtualXORROTW7 MLE not implemented"),
+                else => std.debug.panic("unsupported lookup table index: {}", .{table_index}),
             };
         }
 
@@ -1495,7 +1495,7 @@ pub fn LookupTable(comptime F: type, comptime XLEN: comptime_int) type {
                     else
                         @bitCast(@as(i64, divisor_i32));
                 },
-                else => 0, // Unimplemented tables return 0
+                else => std.debug.panic("unsupported lookup table index for materializeTableEntry: {}", .{table_index}),
             };
         }