🐛 fix(fuzz): compile out replay helpers in fuzz mode

[gaborbernat]

The IDNA, phone and JS harnesses define their file-replay helpers in every build, but only main() calls them. A libFuzzer build with -DTH_IDNA_FUZZ, -DTH_PHONE_FUZZ or -DJM_FUZZ drops main(), so -Werror stops on -Wunused-function for run_builtins and run_file, plus run_dump in the phone harness. #1225 moved the JS entry point behind JM_FUZZ and left its helpers in the same state.

Each harness now opens its #ifdef entry-point switch above the replay helpers, so the libFuzzer build leaves them out and the replay build keeps them. With clang 18, -Wall -Wextra -Werror now builds all six harness and mode pairs, and each libFuzzer binary runs 2,000 inputs clean under ASan and UBSan.

[codspeed]

Merging this PR will regress 1 benchmark

⚡ 1 improved benchmark
❌ 1 regressed benchmark
✅ 579 untouched benchmarks
⏩ 32 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
❌	test_feature[prune-shared-single]	89.1 µs	96.7 µs	-7.88%
⚡	test_feature[shadow-slot-comments]	137.9 µs	110.1 µs	+25.23%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing gaborbernat:fix/harness-fuzz-unused (3a33885) with main (af31f20)}

Footnotes

1. 32 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩