feat: four-corner part color override, cellmap/cell name queries

[Naruto]

Description

Three independent changes, all on the Godot player side.

1. Four-corner part color override. ss_runtime_set_part_color_override_corners has always been in the C API next to the single-color entry point, but only the single-color one was wrapped — a part could not be given a gradient. Adds set_part_color_override_corners(part_name, left_top, right_top, left_bottom, right_bottom, blend_op = 0, priority = 1) and its _by_index twin, passing the colors in the runtime's own lt, rt, lb, rb order.

Both entry points write the same per-part override slot, so the last call wins and the existing clear_part_color_override clears either kind. This is documented in the header, the class reference and the docs site. The color packing shared by both paths moved into pack_color_rgba so the two cannot drift apart.

ss_player.set_part_color_override_corners(
    "body", Color.RED, Color.RED, Color.BLUE, Color.BLUE, 0)

2. get_cellmap_names() / get_cell_names(cellmap_name) on the player. These are the discovery half of set_part_cell_override, and having them on the node saves the caller a round-trip through get_ssab_resource(). They forward to the bound SSABResource and return an empty array when none is assigned; the resource keeps its own copies for enumerating an .ssab that is not on a player yet.

3. Roadmap: the blending / crossfade item is removed. It was filed as blocked on SDK Phase 3, but the SDK shipped the whole-animation blend and crossfade primitives some time ago and the C API has carried them since (ss_runtime_add_blend_source / ss_runtime_crossfade / …). What kept the item from being actionable was never the SDK — the semantics a Godot-facing API would have to commit to (how sources of different lengths relate, whose events fire) are still open. Removed rather than left as a stale blocker; it can be re-filed once that design settles.

Also bumps the ss_player/SpriteStudio-SDK submodule to 427024d (SDK #336, "stop emitting the C++-only headers") — the follow-up to a3af9d5, which switched this repo to the SDK's C headers.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Verification

Build	Result
GDExtension (scripts/build-extension.sh)	pass
Custom module (scripts/build.sh)	pass

Both configurations were built because get_cellmap_names / get_cell_names branch on SPRITESTUDIO_GODOT_EXTENSION (GDExtension returns PackedStringArray directly, the module copies out of Vector<String>), so each branch needed to be compiled at least once.

Not verified: the gradient has not been eyeballed in a running editor. examples/Override_Ringo gained a step that shows it — open that project and press Play to check step 2.

Note that pr.yml is expected to fail at checkout while the SDK submodule is private; this is the known CI blocker, not something this PR introduces.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules