[ASDAlexander77]

What

Under -mm=own, a Shared<T> parameter the body assigns is now copied into a counted local of the same name, as rc does for every assigned parameter and own already did for string parameters.

Why

Own steps aside for Shared handles, so it accepted p = new Shared(..) on a parameter, but the parameter's slot only borrowed the new handle. Assigned in a loop, on one branch, in a try, or before a closure captured it, the handle was released at the end of the block that made it, and a later p.value read freed memory. This was fuzzer finding O4: own crashed (exit 139/134 on Linux with MALLOC_PERTURB_, and a segfault on Windows for the loop form) while gc and rc printed the right value.

Tests

• own_shared_param_assigned covers the loop, branch (taken and not taken), try and closure forms, plus the caller handle's count. It runs under gc, none, rc and own, and under --verify-ownership for rc and own. Without the fix it fails under own with "assigned in a loop".
• Full suite: Windows 5466/5466, Linux 5445/5445. All four fuzzer O4 repros pass under own on Linux.

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