amuleweb: O(N^2) -> O(N) array_push_back via cached scan hint (#666) - #689

Related to #666 — secondary amuleweb-side wedge surfaced by @Stoatwblr's latest gdb on amuleweb after #687 unblocked the amuled side.

Root cause

array_push_back in the PHP engine does:

for (int i = 0; i < 0xffff; i++) {
    PHP_VAR_NODE *arr_var_node = array_get_by_int_key(array, i);
    if (arr_var_node->value.type == PHP_VAL_NONE) {
        return arr_var_node;
    }
}

On a freshly-pushed array of size N, the scan walks i = 0..N-1 (each iteration does an snprintf("%d") + std::map<string, *>::count()) before finding the empty slot at i = N. Building the array via N push_backs costs O(N²).

amule_load_shared → amule_obj_array_create<SharedFileInfo, SharedFile> calls array_push_back once per shared file. On @Stoatwblr's 43585-file install that's ~950M map lookups + string conversions per amuleweb-main-shared.php render, pegging the amuleweb process at 100% CPU for the entire page load.

Backtrace top:

#0 __memcmp_avx2_movbe_rtm
#5 std::_Rb_tree<...>::_M_key_compare ("2051" vs "20509")
#8 std::map<string, PHP_VAR_NODE*>::count
#9 array_get_by_str_key at php_syntree.cpp:676
#10 array_get_by_int_key at php_syntree.cpp:695
#11 array_push_back at php_syntree.cpp:786
#12 amule_obj_array_create<SharedFileInfo, SharedFile> at php_amule_lib.cpp:766
#13 amule_load_shared at php_amule_lib.cpp:787

Fix

Cache the next-push starting index on PHP_ARRAY_TYPE and resume the scan from there, advancing on success. amuleweb's PHP code (engine glue + the default *.php templates) only ever push_backs — no unset, array_pop, array_shift, or array_splice callers — so the hint is always accurate and each push_back collapses to O(1).

For correctness against any future caller that does mixed insert+delete, a fallback low-range scan preserves the original "fill the first hole" semantics if the hint missed.

The anonymous typedef'd struct becomes a tagged struct so the new default-member-init doesn't trip -Wnon-c-typedef-for-linkage.

Test

Local macOS build of amule amuled amulegui amuleweb — clean, no warnings. The bottleneck is purely amuleweb-side, so combined with #687 (amuled-side) the full amuleweb-main-shared.php render path should be free of N² scans end-to-end.