Release notes: 2026-05-27 — wgpu-readback-phase-b¶

Status: Ready for review
Repo: li-langverse/lic
PR: cursor/wgpu-readback-phase-b-5599
PH / REQ: PH-HW viewport, UX-14 native pixels
Author: agent

Summary (one sentence)¶

Phase B wgpu readback scaffold: LIG_WGPU_READBACK=1 selects lig_present_wgpu_readback_paint_summary / native_pixel_source_wgpu_readback (stub) before host present; kid=4 kernel dispatch; studio exit codes 54/55.

Agent continuation (required)¶

Read: docs/game-dev/wgpu-readback-path.md, packages/lig/present/lib.li, runtime/li_rt.c (li_rt_lig_wgpu_readback_stub).
Run: LIC=build/compiler/lic/lic ./scripts/lic-workspace-build.sh packages/li.toml; lic check packages/lig/li-tests/smoke/present_wgpu_readback_stub.li; LIG_HOST_PRESENT=1 LIG_WGPU_READBACK=1 studio vertical smokes.
Then: in-tree wgpu-rs swapchain readback (real GPU texture → CPU RGBA8) — replace stub globals.
Blocked on: wgpu-rs surface in CI images for real GPU readback — not required for phase B scaffold merge.

Changed (specific)¶

Area	What	Evidence
`runtime/li_rt.c` / `li_rt.h`	`li_rt_lig_wgpu_readback_stub`, `li_rt_lig_wgpu_readback_active`, `LIG_WGPU_READBACK`	stub sets source 3
`runtime/li_rt_lig.c`	`li_rt_lig_kernel_run` kid 3 blit, kid 4 readback	kernel bench path
`packages/lig/present/lib.li`	`lig_present_wgpu_readback_paint_summary`, blit fallthrough	`present_wgpu_readback_stub.li`
`packages/li-studio/src/lib.li`	readback branch in `studio_vertical_demo_frame`; exit 54/55	host-present gate
`li-tests/manifest.toml`	`T-PKG-lig-present-wgpu-readback`	manifest row
`docs/game-dev/wgpu-readback-path.md`	Phase B scaffold status + env table	doc

Not changed (scope fence)¶

Real wgpu-rs swapchain / texture readback (phase B future).
trusted.lean axioms.
LLVM LKIR lowering beyond existing blit.lkir stub.
Benchmark threshold / dashboard ingest.

Breaking changes¶

None — li_std_lig_present_version remains 2.

Security¶

N/A — trusted runtime edge only; no new trusted.lean axioms.

Performance¶

N/A — stub only; no bench threshold changes.

Downstream¶

Repo	Action
benchmarks	optional row after real readback benches land
li-render	N/A until swapchain readback

CHANGELOG entry (paste into Unreleased)¶

- **PH-HW viewport phase B:** `LIG_WGPU_READBACK=1` wgpu readback present stub (`native_pixel_source=3`, kid=4); studio vertical demo exit 54/55 — [2026-05-27-wgpu-readback-phase-b.md](docs/release-notes/2026-05-27-wgpu-readback-phase-b.md).