fix: release transfer-full GObject return refs so they can be GC'd

[romgrk]

Fixes #446.

Problem

GObjects returned from GI functions — constructor helpers like Type.new(), and transfer-full returns generally — were never collected, even after every JS reference was dropped and GC was forced repeatedly. The same type built with new Type() was collected correctly, so the leak was specific to the GI-return path. The reporter hit a monotonically growing live heap (and increasingly long major-GC pauses) from a git poll creating Ggit objects via .new() on a timer.

construction	created	alive after dropping all refs + GC×8
Gio.Menu.new() (GI function)	10000	10001 → 2 (fixed)
new Gio.Menu() (JS constructor)	10000	2

Root cause

A transfer-full return hands node-gtk an owning reference on top of the wrapper's toggle ref. node-gtk only needs the toggle ref, so the extra reference must be released; otherwise the refcount never falls back to 1, ToggleNotify never flips the V8 handle to weak (SetWeak), and the object (and its wrapper) is pinned alive for the lifetime of the process.

• new Type() works because GObjectConstructor drops its construction ref.
• OUT GObject args work because FreeGIArgument already drops their transfer-full ref.
• The return value is never freed on the success path, so it leaked 1:1 and unbounded.

Fix

After wrapping the return value, drop the extra owning reference for a transfer-full GObject return — the return-value counterpart of the OUT-arg balance and of RefObjectForTransferFullIn (the #439 IN-side fix).

Ownership is sampled before wrapping, since associating the wrapper sinks any floating reference. The helper guards with G_IS_OBJECT (excludes GParamSpec / boxed / fundamental interface types) and !g_object_is_floating (a sunk floating ref is nothing extra to drop), so floating widgets and transfer-none returns are left untouched.

Verification

• Issue's exact repro: Gio.Menu.new() goes from 10001 alive → 2 alive, matching new Gio.Menu().
• New regression test tests/object__gi_return_leak.js (creates many Gio.Menu.new(), drops refs, GCs, asserts the wrappers are reclaimed via FinalizationRegistry).
• Full suite: 84 passing including the new test. The only failure (require.js) is a pre-existing, environment-specific typelib conflict (PeasGtk / GIRepository 3.0 vs 2.0) that fails identically on unmodified master.

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