Phase 6 — 2D Render v2

Authoritative spec. See also the phase index, perf-dod, and testing-strategy. Live issue numbers: pinned master #810.

Phase 6 — 2D Render v2 (W4) — concurrent with Phases 5, 7

Goal: Four sprite renderers become one instanced core; 2D camera and culling move engine-side. 8 issues.

ID	Title	Grp	Effort	Blocked by
ENG2-P6-01	RFC-0008: Unified instanced 2D sprite core	—	S	Phase 4 gate

Four parallel renderers today: immediate FFI (~15 GL-state calls/sprite, ffi/renderer/draw/internal.rs:107-132), batched FFI (full rebuild/sort/upload per call, draw/batch.rs:171-343), retained ECS SpriteBatch (unreachable from FFI, rendering/sprite_batch/batch.rs), WASM batcher (wasm/sprite_renderer/renderer_core.rs). Zero GPU instancing anywhere — every sprite CPU-expanded to 4 vertices.

ID	Title	Grp	Effort	Blocked by
ENG2-P6-02	Instanced sprite core: static unit quad + dirty-tracked per-instance buffer (incl. flip flags, #424)	—	L	P6-01

Per-instance {transform, size, uv-rect, color, tex-index, flags}; persistent buffers, changed-only uploads (replaces batch.rs:234-242 CPU corner rotation + batch.rs:355-417 per-frame vertex gen). Reuses Phase 2 instancing patterns behind the backend trait.

ID	Title	Grp	Effort	Blocked by
ENG2-P6-03	Route FFI sprite paths through the core; delete immediate + rebuild-per-call paths	A	M	P6-02
ENG2-P6-04	Unify retained ECS SpriteBatch + WASM batcher onto the core; delete duplicates	B	M	P6-02
ENG2-P6-05	First-class 2D camera (view+proj in shader) + engine-side 2D culling + coordinate-origin option (#554)	C	M	—

P6-05: FFI batch sets only u_viewport (draw/batch.rs:355); Throne does world→screen math + visibility in C# per sprite (GoudRenderPort.cs:275). Camera uniform + grid/quadtree culling before submission; #554’s configurable origin lands in the camera.

ID	Title	Grp	Effort	Blocked by
ENG2-P6-06	Chunked tilemap renderer from TileLayer gids	A	M	P6-02
ENG2-P6-07	Auto atlas/texture-array integration with the sprite core	B	M	P6-02
ENG2-P6-08	Shaped-text run cache + S4 sprite-storm gate validation	C	M	P6-03

P6-06: assets/loaders/tiled_map/layer.rs:22 TileLayer is raw Vec<u32> gids with no renderer — the direct cause of per-tile DrawSprite in 2D consumers.
P6-07: ShelfPacker + full atlas FFI exist (rendering/texture_atlas/packer.rs, ffi/renderer/atlas/ffi.rs) but nothing auto-packs; collapse many textures into few draws without consumer bookkeeping.
P6-08: ffi/renderer/text/draw_impl.rs:45 re-shapes every string every frame (atlas is cached, shaping is not); cache runs keyed (string, font, size).

S4: 100k sprites / 10k visible ≥ 60 FPS; ≤ 20 draw calls; CPU submit ≤ 2 ms.
Exactly one sprite renderer remains (lint gate: immediate path + duplicate batchers deleted; wasm routes through the core).
2D example runs with zero consumer-side world→screen math.
200×200 tile layer renders in ≤ 4 draw calls.