refactor(runner): split src/runner.sh into a src/runner/ module

[Chemaclass]

Summary

src/runner.sh is 2145 lines and 57 functions covering five distinct
responsibilities (.claude/rules/architecture-map.md:51 already names them: "file
loop, per-test execution, retry/timeout, result parsing, failure context"). Split it
into a src/runner/ module behind a thin aggregator, following the existing
src/assertions.sh → src/assert_*.sh pattern.

Blocked by #923 — path-keyed embed markers must land first, or
src/runner/parallel.sh collides with src/parallel.sh and breaks
test_built_binary_embeds_each_src_file_exactly_once.

This is a pure move

The namespace stays bashunit::runner::*. No function is renamed, no call site
changes. Every hunk should be a verbatim relocation. If a behavior fix looks
necessary along the way, open a separate issue for it — do not fold it in, or the
diff stops being reviewable.

bashunit:90 keeps its single source "$BASHUNIT_ROOT_DIR/src/runner.sh".
src/runner.sh becomes an aggregator holding only source lines and comments
(required — see the aggregator rule in #923).

Target layout

file	~lines	functions
src/runner.sh	14	aggregator only
src/runner/context.sh	145	restore_workdir, sync_coverage_flag, export_test_identity, resolve_test_location, apply_interpolated_title, source_login_shell_profiles, needs_test_duration, _supports_reliable_pipefail
src/runner/payload.sh	215	the 13 _BASHUNIT_RUNNER_*_OUT slot globals + extract_encoded_field, compute_total_assertions, extract_subshell_type, format_subshell_output, decode_subshell_output, extract_result_counts, is_simple_progress_output, line_exists_in_output, extract_assertion_runtime_output
src/runner/diagnostics.sh	135	detect_runtime_error, classify_kill_signal, halt_if_stop_on_failure, record_profile, print_verbose_test_summary, render_running_file_header
src/runner/result.sh	215	parse_result{,_sync,_parallel}, get_failure_source_context, write_{failure,skipped,incomplete,risky}_result_output
src/runner/parallel.sh	90	_supports_wait_n, _count_running_jobs, wait_for_job_slot, spinner
src/runner/hooks.sh	380	run_set_up{,_before_script}, run_tear_down{,_after_script}, execute_{file,test}_hook, record_{file,test}_hook_failure, clear_mocks, clean_script_test_functions, clean_set_up_and_tear_down_after_script, cleanup_on_exit
src/runner/provider.sh	120	parse_data_provider_args
src/runner/exec.sh	450	run_test, execute_test_body, run_with_timeout, build_timeout_result, call_test_functions
src/runner/discovery.sh	280	load_test_files, functions_for_script
src/runner/bench.sh	100	load_bench_files, call_bench_functions

The dependency layering is acyclic — 53 of 57 functions are leaves; only
load_test_files, load_bench_files, run_test and parse_result have callees:

context · payload · diagnostics  →  parallel · hooks · result  →  provider · exec  →  discovery · bench

src/runner/parallel.sh deliberately shares a basename with src/parallel.sh; that
is the case #923 makes safe.

Tests: keep them flat

Do not create tests/unit/runner/. make test globs
$(wildcard $(TEST_SCRIPTS_DIR)/*/*[tT]est.sh) — one level only (152 files vs 156
recursive). Anything at tests/unit/runner/payload_test.sh is silently skipped
by make test, which CI runs (.github/workflows/tests.yml:29, :90). Deepening
the glob is not an option either: the 4 extra recursive matches are deliberately
excluded fixtures (tests/unit/fixtures/tests/example1_test.sh and friends).

So split tests/unit/runner_test.sh (430 lines) into flat
tests/unit/runner_<area>_test.sh files mirroring the src modules — the same
convention coverage_*_test.sh already uses (9 files).

Watch out for

• File-scoped ShellCheck directive. src/runner.sh:2 is
  # shellcheck disable=SC2155, which currently covers the whole file. Carry it
  only to the new files that actually need it, or narrow it to specific lines.
  Do not blanket-apply it to all ten.
• Return-slot globals now cross files. src/runner/payload.sh declares the 13
  _BASHUNIT_RUNNER_*_OUT slots; src/runner/exec.sh reads them. Expect SC2034 /
  SC2154 noise and resolve it the way src/state.sh:16-17 already does (a scoped
  disable with a comment naming the reader), not with a blanket file-level disable.
• First line must be the shebang. build.sh's tail -n +2 strips line 1 of
  every embedded file.
• jobs -pr must stay in the dispatcher shell. wait_for_job_slot and
  _count_running_jobs depend on running in the shell that spawned the workers.
  Moving them to another file is fine; wrapping them in a subshell is not.
• _BASHUNIT_RUNNER_RESULT_ORDINAL is read outside the runner —
  src/coverage.sh:224. It stays a plain global; do not scope it.
• No new forks. .claude/rules/perf-fork-budget.md budgets are enforced by
  tests/acceptance/bashunit_{coldstart,run}_forks_test.sh on three platforms.
  A file split adds no forks (parse time dominates cold start, file opens don't) —
  keep it that way.

Docs to update

• .claude/rules/architecture-map.md:51 — the single runner.sh row becomes the
  module table
• .claude/rules/architecture-map.md:17 — the call-flow annotation (runner.sh: the per-file loop) → src/runner/discovery.sh
• .claude/rules/bash-style.md:121 — the return-slot example points at
  src/runner.sh; retarget to src/runner/payload.sh
• New adrs/adr-010-*.md using adrs/TEMPLATE.md: why the module split, why the
  aggregator pattern, why tests stay flat

Acceptance criteria

• git diff shows only relocations — no renamed functions, no changed logic
• bashunit:90 still sources exactly one runner file
• src/runner.sh contains only source lines and comments
• Every new file's first line is #!/usr/bin/env bash
• ./bashunit tests/ green
• ./bashunit --parallel tests/ green
• make sa && make lint green
• bash build.sh bin -v prints ✅ Build verified ✅
• Fork-budget acceptance tests green
• tests/unit/runner_test.sh split into flat per-area files, all picked up by
  make test/list
• Bash 3.0+ compatible
• No CHANGELOG entry — internal, no user-visible behavior change
• ADR added

Do not

• Do not rename any function or change any behavior
• Do not create tests/unit/runner/ (see above)
• Do not touch src/coverage.sh — separate issue
• Do not run shfmt -w; make lint is the format gate

[Chemaclass]

Done in #927 (merged).