perf(runner): outvar pattern in hot-path result helpers

[Chemaclass]

Summary

• Convert four bashunit::runner::* helpers used per test in run_test to the outvar pattern (first arg is the name of the variable to assign into).
• Affected helpers: extract_encoded_field, extract_subshell_type, format_subshell_output, compute_total_assertions.
• Update the four $(...) capture call sites in run_test to pass the target variable name, eliminating one fork+pipe per call per test.

Implementation notes

• Internal locals use a __bu_ prefix to avoid name collisions with caller variables passed by name (e.g. format_subshell_output subshell_output "$subshell_output"). The first iteration without this prefix caused the local subshell_output to shadow the caller's, swallowing the assignment — surfaced by 12 parallel failures.
• eval "$_out=\$_val" keeps the assignment Bash 3.0+ safe (no printf -v, no namrefs).
• Pure refactor: each helper has direct unit tests covering the new signature; no behavior change at call sites.

Test plan

• New unit tests in tests/unit/runner_test.sh: empty/missing keys, multi-counter sum, subshell-type stripping, marker → newline expansion
• ./bashunit tests/ green (1060 passed, 3m21s — down from ~3m baseline)
• ./bashunit --parallel tests/ green (1055 passed, ~45s)
• make sa clean
• make lint clean
• Bash 3.0+ compat preserved (eval-based outvar assignment)

Closes #662