bug(parallel): same-named test files in different directories silently lose results

[Chemaclass]

Summary

Under --parallel, per-test results are bucketed into a directory named by the test file's
basename. Two test files with the same basename in different directories share that bucket,
their per-suite ordinals collide, and one file's results overwrite the other's.

The run stays green and simply reports fewer tests. Nothing errors.

Same class of defect as #923, which fixed exactly this in build.sh — the fix there was to key
on the repo-relative path instead of the basename.

Root cause

src/runner/result.sh:28-32:

local test_suite_base="${test_file##*/}" # basename
local test_suite_dir="${TEMP_DIR_PARALLEL_TEST_SUITE}/${test_suite_base%.sh}"
[ -d "$test_suite_dir" ] || mkdir -p "$test_suite_dir"

local unique_test_result_file="${test_suite_dir}/${_BASHUNIT_RUNNER_RESULT_ORDINAL}.result"

tests/a/foo_test.sh and tests/b/foo_test.sh both resolve to <tmp>/foo_test/. The ordinal
is unique per suite, not globally, so <tmp>/foo_test/1.result is written twice and the
first write is lost.

Reproduce

Measured while mirroring tests/unit/ onto the src/ module layout (#957), which produced
three duplicate basenames (helpers_test.sh, json_test.sh, doubles_test.sh each in two
directories):

	--parallel --simple --strict total
with 3 duplicate basenames	1536
after renaming them unique	1588

52 tests silently vanished. Reproducible, not flaky — three consecutive runs each.

Sequential mode reports the full count either way, so the two modes disagree and only the
parallel one is wrong.

Minimal case:

mkdir -p t/a t/b
printf '#!/usr/bin/env bash\nfunction test_a() { assert_same 1 1; }\n' > t/a/dup_test.sh
printf '#!/usr/bin/env bash\nfunction test_b() { assert_same 1 1; }\n' > t/b/dup_test.sh
./bashunit t # 2 tests
./bashunit --parallel t # fewer

Impact

Affects any user project whose test tree has two same-named files in different directories
— a normal layout when tests mirror a source tree, which is precisely what the framework's own
docs encourage. Results are lost silently, so a failing test can disappear and CI stays green.

Proposal

Key the suite directory on the repo-relative path with separators sanitized, not the
basename — the #923 fix, applied here. bashunit::helper::generate_id already sanitizes a
path into a variable-safe token and could be reused, but note the comment above this code:
it is on the per-test parallel path and deliberately fork-free, so the derivation must stay
pure parameter expansion.

Something of this shape, keeping the existing no-fork property:

local test_suite_base="${test_file#./}"
test_suite_base="${test_suite_base%.sh}"
test_suite_base="${test_suite_base//\//_}"

Acceptance criteria

• RED first: a test asserting two same-named test files in different directories both
  report their results under --parallel
• ./bashunit --parallel tests/ and ./bashunit tests/ report the same total on a
  tree containing duplicate basenames
• No new fork on the per-test path (.claude/rules/perf-fork-budget.md); the fork-budget
  acceptance tests must stay green
• Bash 3.0+ compatible
• ./bashunit tests/ · --parallel · --parallel --simple --strict · make sa ·
  make lint · bash build.sh bin -v green
• CHANGELOG entry under Unreleased → Fixed — this is user-facing

Do not

• Do not fix it by forbidding duplicate basenames; mirroring a source tree makes them normal
• Do not add a mktemp/basename fork to the per-test path