Fix 5 regvm test failures: parameter tailcalls, set-car!/substring emit, remainder opcode

- Add parameter object handling to ROPTAILCALL dispatch (fixes parameterize, which broke test-mcp-server and any code calling parameters in tail position) - Fix set-car!/set-cdr! register emission: use arg registers instead of dest register for the pair operand (fixes test-gc-stress) - Fix substring (ternary op) result not moved to dest register after execution - Add ROPREM opcode for remainder operation (was missing from regvm entirely) - Add (sigil string) import to test-ansi, test-semver, test-deps for string-contains? which requires explicit import - Remove orphaned test-execute.sgl (module deleted in 5e65b1ab)