perf(runner): cache data-provider lookup per file instead of grep+sed per test

[Chemaclass]

Part of #761.

Problem

bashunit::runner::call_test_functions calls bashunit::helper::get_provider_data "$fn_name" "$script" for every test function (src/runner.sh:778-782). get_provider_data (src/helpers.sh:295-319) runs grep -B 2 -E ... "$script" | sed -nE ... — two external forks plus a full re-read of the test file from disk — just to discover whether a # @data_provider annotation exists above the function. For the overwhelming majority of tests there is no provider.

Measured cost: ~3.3ms per test on macOS bash 3.2, ~4s across the suite, compounding inside the 258 nested ./bashunit acceptance runs.

Task

Scan each test file once (when its functions are enumerated in call_test_functions, src/runner.sh:757 area) and build a test-function -> provider-function map for that file; per-test lookup becomes pure bash.

Implementation sketch (Bash 3.0 compatible — no associative arrays):

• One grep/awk pass per file extracting pairs of (annotation provider name, following test function name). Must match the same things the current probe matches: # data_provider fn and # @data_provider fn (see sed pattern at src/helpers.sh:313), annotation up to 2 lines above a function defined either as function test_x or test_x().
• Store as a newline-delimited global string of test_fn provider_fn records (or two parallel indexed arrays); reset it per file. Lookup with a plain loop or case match.
• Keep bashunit::helper::execute_function_if_exists behavior for missing provider functions.
• Preserve the issue Data providers don't work when changing directory in set_up_before_script #529 fallback: if the script path is not readable, retry with $BASHUNIT_WORKING_DIR/$script (src/helpers.sh:301-303), and return no providers if still unreadable.
• get_provider_data can remain as a thin wrapper over the cache so its unit tests keep passing, or be replaced — follow whichever keeps the public surface unchanged.

Tests first (TDD)

• Existing provider behavior tests: tests/functional/provider_test.sh must pass unchanged (both sequential and --parallel).
• Add unit tests for the new scanner: file with multiple providers, provider with @ and without, annotation 2 lines above the function, two functions sharing one provider, file with none, unreadable path (issue Data providers don't work when changing directory in set_up_before_script #529 case).

Constraints

• Bash 3.0+ only: no declare -A, no [[ ]], no ${var,,}, no negative array indexing, no &>>.
• Return values on the hot path must follow the outvar/global-slot pattern from .claude/rules/bash-style.md — no new $(...) per-test call.
• Data-provider execution order and row order must not change (test names include interpolated provider data).

Acceptance criteria

• ./bashunit tests/ and ./bashunit --parallel tests/ pass.
• make sa, make lint pass; shfmt -w . produces no diff.
• Fork census shows the per-test grep+sed pair replaced by at most one scan per file.
• Before/after timing of the 100-noop benchmark reported in the PR.

Line references valid at commit 4d80e7c.