feat: configurable viewport, fused ARGB raster, and Vue Vapor host hardening for ESP32-P4

[odex21]

Motivation

ESP32 devices ship with wildly different panels — 480×272, 640×226, 800×480, etc. — there's no "standard" size like a PSP screen. PocketJS currently hardcodes 480×272 in the rasterizer, the wasm binding, and both hosts. This PR removes that assumption, then adds the last few pieces needed to run on real ESP32 hardware and iterate in the browser preview.

Each commit is self-contained and reviewable independently.

Commits

1. feat(core,wasm,host): configurable viewport for layout and raster — Ui carries a viewport; render_scaled derives framebuffer dimensions from it instead of the fixed SCREEN_W/H; wasm exposes ui_set_viewport; web/sim hosts accept viewport and raster-density options. Any target can size its logical surface at boot without touching the core. Stock hosts keep the 480×272 golden contract.

2. feat(raster): fused ARGB8888 output for ESP32-P4 PPA — new render_scaled_argb() emits A,R,G,B bytes — the in-memory order the ESP32-P4 PPA SRM consumes as ARGB8888. Byte placement is fused into blend_px via a const generic shared with the RGBA path, so it costs nothing; the firmware drops its per-frame RGBA→ARGB CPU reorder copy (measured: present 25.5 → 13.0 ms, 14 → 17.1 FPS).

3. fix(vue-vapor): isolate the guest document and tolerate fragment-wrapper mirrors — covers two issues encountered while hosting a Vue Vapor player page:

   • Document isolation: when PocketJS is embedded in a real browser page, globalThis.document already exists. The old if (!g.document) guard never fires, so the PocketJS DOM facade is not installed, and the Vue Vapor guest's document.createElement() falls through to the real browser DOM. This PR introduces globalThis.__pocketDocument and aliases document to it via esbuild define (only for --framework=vue-vapor builds), so the real document stays untouched.
   • Fragment-wrapper tolerance: Vue Vapor injects internal fragment wrappers ({ nodes: […] }) into node.children during dynamic branch swaps (e.g. {playing ? <PauseIcon/> : null}). Upstream PR chore: upgrade Vue to 3.6.0-rc.1 #137 and fix(vue-vapor): guard node.children access #142 add if (node.children) guards that prevent crashes when children is undefined — but a fragment wrapper passes the truthy guard and is silently serialized as {"t":"undefined"}, losing all inner content. forEachTreeChild / forEachTreeMirror recursively unpacks these wrappers so tree snapshots stay correct.

Why forEachTreeChild instead of if (node.children)?

PR #137 (merged) and #142 (open) add null guards to serializeNode / countNodes / subtreeHasRetained. These prevent the crash but don't handle the fragment-wrapper case. A test added in this PR demonstrates the difference:

// Inject a Vue Vapor fragment wrapper into the tree
(playbackStatus.children as unknown[]).push({ nodes: [[pausedTextNode]] });
// Trigger tree serialization
push({ t: "getTree" });
frame();
expect(treeJson).toContain("PAUSED"); // ← fails without forEachTreeMirror

With the simple guard: wrapper passes through → serialized as {"t":"undefined"} → test fails.
With forEachTreeChild: wrapper is recursively unpacked → "PAUSED" found → test passes.

This PR keeps the comment-anchor skip from #137 — forEachTreeChild subsumes both.

Compatibility

• render / render_scaled output is untouched and byte-exact: 49/49 committed pixel goldens pass.
• cargo test (core): 78/78, including a new test pinning ARGB↔RGBA channel mapping through the translucent destination-read blend path.
• bun test: renderer / clock / tiles / cursor / devtools / vue-vapor-dom 88 pass; sim 7 pass; launcher-sim 10 pass.
• No new dependencies, no feature flags, no breaking API changes.

Hardware validation (commit 2)

ESP32-P4 @ 640×226 → PPA 2× scale + 90° rotation → 452×1280 panel: present time 25.5 → 13.0 ms, whole-pipeline 14 → 17.1 FPS, pixel stream bit-identical to the previous software reorder (colors verified on device).

[odex21]

Updated PR description

The PR body was revised to address review feedback. Key changes:

Removed:

• Duplicate rebase notes (the "package.json test chain is a union" and "src/devtools.ts" paragraphs appeared twice)
• The individual "feat(host-web): viewport URL params, pointer touch input, perf HUD" commit was folded into the viewport commit description — its content is still in the diff, just not called out separately since it's integral to the viewport feature

Added:

• A dedicated section "Why forEachTreeChild instead of if (node.children)?" that explains the difference from the upstream guard pattern in chore: upgrade Vue to 3.6.0-rc.1 #137 and fix(vue-vapor): guard node.children access #142
• A concrete fragment-wrapper test snippet showing the failure mode with the simple guard
• Clearer separation between the two halves of commit 3 (document isolation vs fragment-wrapper tolerance)

Unchanged:

• All three commits, their code, and their commit messages are untouched
• Hardware benchmarks and compatibility numbers are the same