perf(runner): reduce subshells in hot path

[Chemaclass]

Context

src/runner.sh contains ~98 $(...) command substitutions. Many sit in the per-test hot path, forking a subshell on every test execution. Suite runs ~80s sync / ~22s parallel; subshell overhead is a sizable chunk.

Hot-path offenders

• bashunit::runner::extract_encoded_field (src/runner.sh:49) — called per field per test
• bashunit::runner::extract_subshell_type (src/runner.sh:78)
• bashunit::runner::format_subshell_output (src/runner.sh:84)
• bashunit::runner::compute_total_assertions (src/runner.sh:62)

Param expansion already used in places — push further. Common pattern:

foo=$(bashunit::runner::extract_subshell_type "$out")

Refactor to fill an outvar (Bash-3 safe via eval or namref-free assignment) instead of capturing subshell stdout:

bashunit::runner::extract_subshell_type out "$subshell_output"
# function body: eval "$1=\"\${2%%]*}\""; eval "$1=\"\${$1#[}\""

Or inline the param expansions at call sites where the helper is one-liner.

Acceptance

• No $(...) capture in per-test hot path of runner.sh for the listed helpers
• All tests green (./bashunit tests/)
• Parallel tests green (./bashunit --parallel tests/)
• Bash 3.0+ compat preserved (no namrefs, no declare -n)
• Benchmark before/after wall-clock on tests/unit/ posted in PR

Notes

• Pre-compiled regex _BASHUNIT_RUNNER_PARSE_RESULT_REGEX (runner.sh:5) already exists — extend usage if it reduces helper calls.
• Validate with PS4='+ \$EPOCHREALTIME ' bash -x ./bashunit tests/unit/<file>.sh 2>trace to confirm hotspots pre/post.