Harden allocation size checks

[falseywinchnet]

Motivation

• Prevent size_t arithmetic overflow in heap growth and aligned allocation paths that could produce oversized allocations or wraparound on 64-bit/32-bit hosts.
• Avoid integer/cast overflows and out-of-range indexing in STFT plan creation where long long/int casts and buffer-size math are used.
• Ensure C API surfaces allocation failures via bfft_status instead of letting C++ exceptions escape into C callers.

Description

• In src/detail/bruun_kernel.hpp the heap_array::push_back growth logic now computes a safe new_cap and returns false when doubling would overflow, replacing the unchecked cap_ * 2 path.
• In src/detail/bruun_kernel.hpp the aligned_alloc_raw routine now checks for sizeof(T) * count overflow and guards the padding addition against overflow before calling posix_memalign/_aligned_malloc.
• In src/stft.cpp added add_overflows_size and mul_overflows_size helpers and several precondition checks (bounds against long long/int limits and addition/multiplication overflow) in bfft_stft_plan_create to reject pathological inputs.
• In src/stft.cpp plan/vector allocation and setup are wrapped in a try block so std::bad_alloc maps to BFFT_ERROR_ALLOCATION and other exceptions map to BFFT_ERROR_INTERNAL, preserving the C ABI semantics.

Testing

• Ran make test which built the library and executed tests/correctness and tests/api_c, and both reported success.
• Ran the stricter build/check make check-cxx17 (with -Werror) which completed successfully.
• No test failures observed after the changes.

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Codex Task