[ASDAlexander77]

The first PR of -mm=own phase 6 (corpus report and diagnostics polish). It is an rc-side fix, so it stands alone and can merge on its own.

The bug

When top-level code declares a global (const t = new C(), or let/var), its initial value is stored into the global with no reference of its own. The end-of-block release (§9.30) then gives the value's reference back when the top-level code ends, so the global points at a freed block. 5ai fixed this for an assignment to a global (t = new C(), through isOwnedGlobalSlot), but not for the declaration's store (MLIRGenImpl.h, the "not global scope" branch).

The fix

The declaration's store now takes the owning-slot path, the same one an assignment takes:

• it takes over an owned reference, or retains;
• it releases what the slot held before, which matters for a var met again in a loop. A global starts at zero, so the first release does nothing.

This doesn't branch on the memory model in MLIRGen (spec §4.3): gc erases the retain.

Why it matters

• rc: not observable today. Nothing runs after a program's top-level code, and a module with top-level code can't be imported (below). The test's teeth check confirms this: with the fix reverted, rc still passes.
• own: the store reads as a move and the release as a second reference. That made 28 corpus files fail with the catch-all "takes a second reference"; 8 of them now compile.

Found, not fixed (every model, outside this PR)

• A module with a top-level statement (print(...)), imported as source, fails with "Global is referenced by parentless instruction! @puts" (invalid LLVM IR).
• With -shared, an exported const from such a module can't be resolved by the importer ("can't resolve name").

Verification

• Release suite: 3399 of 3400 pass. The one failure, test-compile-gc-defaultlib-collector, is the stale installed default library, as on -mm=own phase 5a: a closure that does not escape borrows what it captures #436-Builtin iterator methods without the default library: a.map(f).map(g) #443. The rc suite is unchanged.
• WSL/GCC: builds, and the 2056 tests run there pass (every own test, the rc and none corpus, and the new test).
• Own corpus: 364 → 372 of 591, none lost. Each new file passes test-runner -mm=own, AOT and JIT, with gc's output (import_class_interface.ts as its export_ pair; its non--shared JIT mode is unsupported under every model): 00class_from_tuple, 00class_indexer, 00class_protected_constr, 00if_conditional_compile, 00safe_cast_typeof, 00toplevel_control_flow, 00toplevel_no_main_with_helpers, import_class_interface.
• Teeth: with the fix reverted, the test fails under own (AOT, JIT and the no-counting check).

Test

own/own_global_script.ts runs under own, rc, none and gc. It covers const, let and var globals in top-level code (a class and a string), a var in a nested block, reads from a function after the heap is churned, and an overwrite.

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