Value-initialize CompiledInstr::opcode

cppcheck (uninitMemberVarNoCtor) flags the scalar `opcode` member as
having no initializer while its sibling container members self-init.
Every real construction aggregate-initializes all fields, so this was
never a live bug, but value-initializing it gives a default-constructed
instr a defined opcode and keeps the check clean.

Surfaced by the brew cppcheck (2.21) in CI, which enables this check
where the older local 2.13 did not.

Author: claude

c_src/emily/program.hpp

@@ -53,7 +53,10 @@ inline int64_t index_of(int64_t r) { return r & kIndexMask; }
 class Program;
 
 struct CompiledInstr {
-  Opcode opcode;
+  // Value-initialized so a default-constructed instr has a defined opcode
+  // (the other members are containers that self-initialize). Every real
+  // construction aggregate-initializes all fields, overriding this.
+  Opcode opcode{};
   std::vector<int64_t> operands;             // packed refs
   std::vector<std::vector<int64_t>> iattrs;  // integer attrs (shapes/axes/dtype codes)
   // Nested programs an instruction carries (empty for all but control