replace `socialmixr::contact_matrix()` with piped `socialmixr::symmetrise()`

**Is your feature request related to a problem? Please describe.**

similar to #152, to replace `socialmixr::contact_matrix()` with a full transition to the piped workflow by latest `{socialmixr}` after https://github.com/epiforecasts/socialmixr/issues/330

**Describe the solution you'd like**

``` r
library(socialmixr)
library(magrittr)
library(wpp2024)

data("popAge1dt")

uk_demo <- popAge1dt %>%
dplyr::filter(name == "United Kingdom", year == max(year)) %>%
dplyr::transmute(age = socialmixr::limits_to_age_groups(x = age), population = pop * 1000) %>%
socialmixr::rebin_ages(age_limits = c(0, 20, 40))

contact_data3 <- polymod[country == "United Kingdom"] %>%
socialmixr::assign_age_groups(age_limits = c(0, 20, 40)) %>%
socialmixr::compute_matrix() %>%
socialmixr::symmetrise(survey_pop = uk_demo)

contact_matrix3 <- contact_data3$matrix
demography_vector3 <- uk_demo$population
names(demography_vector3) <- rownames(contact_matrix3)

contact_matrix3
#> contact.age.group
#> age.group [0,20) [20,40) [40,Inf)
#> [0,20) 7.883663 3.105519 3.290997
#> [20,40) 2.806513 4.854839 4.986325
#> [40,Inf) 1.488256 2.495156 5.005571
demography_vector3
#> [0,20) [20,40) [40,Inf)
#> 15962345 17662976 35297722
```

^{Created on 2026-08-21 with [reprex v2.1.1](https://reprex.tidyverse.org)}

**Additional context**
Add any other context or screenshots about the feature request here.

Search the tutorials for uses of socialmixr::contact_matrix() and read the surrounding workflow before changing them. Apply the piped assign_age_groups(), compute_matrix(), and symmetrise() sequence shown in the issue, then verify that the resulting contact matrix and demography vector match the demonstrated outputs.

Written by the indexing model from the issue text.