fix: drive Instance parts nested inside Instance parts (SDK bump)

[Naruto]

What

An Instance inside an Instance never advanced. _update_instance_children — the
sim phase — was reached only from _seek_and_redraw, and a parent-driven child
never enters it: update() returns early on _parent_driven, and the child's
draw arrives straight from _redraw_child_if_frame_changed. So everything below
the first level held the frame _fetchAnimation's one-off setup left it at,
however far the animation ran.

Also bumps SpriteStudio-SDK to the landed
cri-middleware/SpriteStudio-SDK#399, which fixes an Instance key with a
startOffset losing the tail of its sub-animation.

Player-facing effect

• Nested sub-animations play. A project whose Instance parts reference packs
  that themselves hold Instance parts (an Adobe Animate conversion typically
  nests three or four deep) previously showed those inner parts frozen on their
  setup frame. They now step with their parent.
• A phase-offset Instance runs its whole cycle. With the SDK change, a key
  carrying startOffset keeps its section instead of collapsing to two frames —
  a walk cycle whose second leg is half a cycle out of phase walks instead of
  vibrating in place.
• Clipping carried out of a nested Instance reaches the right parts, because
  the carry-over flag (visible_this_frame) is now settled before the child
  draws.

Changes

• ss_player/ss_internal_player.cpp
  • _drive_instance_slot recurses into child->_update_instance_children
    ahead of the child's draw.
  • Hiding an Instance child moves out of the sim phase and into
    _drawAnimation, beside the Effect emitter canvas items. The draw is what
    re-shows them, and a player holding a frame does not redraw — hiding in the
    sim phase stranded the children of every held frame with nothing to switch
    them back on. That surfaced as a sub-animation blanking out whenever a seek
    landed on a fractional frame with sub-frames off, which the Godot editor's
    seek bar produces.
• ss_player/SpriteStudio-SDK — submodule bump to the merged #399.

Verification

• scripts/run-tests.sh — 35 cases, 101 assertions, 0 failed, 0 skipped, on the
  GDExtension build.
• Both builds compile: scripts/build.sh (custom module) and
  scripts/build-extension.sh.
• Rendered before/after on an instance-heavy Adobe Animate conversion: identical
  frames differ where the nested instances now advance, and with sub-frames off
  a fractional seek renders byte-identical to its integer frame
  (24.0 / 24.25 / 24.5 / 24.75 one hash; 269.5 = 269.0, 270.5 = 270.0,
  271.5 = 271.0, 430.5 = 430.0). With sub-frames on, fractional frames
  interpolate as before.
• Stage boundaries still swap cleanly — one Instance visible at a time, no
  ghosting.

Known limit, not addressed here

_needs_continuous_update() looks one level down, so an independent Instance
nested two or more levels deep loses the ticks where the root holds its frame
and runs slow. Nothing in the corpus to hand carries one — every Instance key
across the projects checked is synced — so it is left until an asset shows it.