feat(bsn): Bevy 0.19-rc migration + BSN authoring (buiy_bsn)

[intendednull]

What

Brings real Bevy Scene Notation (bsn!) authoring to Buiy. BSN's baseline landed upstream via PR #23413 (milestoned 0.19, in bevy_scene), so this bumps the workspace Bevy 0.18.1 → 0.19.0-rc.3 and adds a thin buiy_bsn authoring layer.

Spec: docs/specs/2026-06-18-buiy-bsn-integration-design.md · Plan: docs/plans/2026-06-18-bevy-0.19-bsn-migration.md · Research: docs/reports/2026-06-18-bevy-0.19-bsn-migration-research.md

⚠️ Merge note — release-candidate dependency

This pins Bevy 0.19.0-rc.3 (no 0.19 stable exists yet), a scoped, deliberate exception to the rolling-latest-stable policy (foundation architecture §2.9) — bsn! is unreachable on any stable Bevy. The exception closes when 0.19.0 stable ships (tracked in follow-ups.md). This is the reviewer's call to make — flagging the rc pin explicitly.

The migration (Bevy 0.18→0.19, wgpu 27→29, accesskit 0.21→0.24)

• Render graph removed upstream → the old BuiyNode: ViewNode is now a single buiy_pass system in the Core2d schedule (Core2dSystems::EarlyPostProcess). One system = one command encoder = byte-identical to the old single ViewNode (preserves cross-pass LoadOp::Load composite loads). Paint order/correctness unchanged.
• wgpu fields (multiview_mask, MipmapFilterMode, immediate_size); ECS Replace→Discard; accesskit TreeUpdate.tree_id.
• 0.19 panics (vs 0.18 skip) on a missing Res → harnesses init the SkinnedMeshInverseBindposes asset that MeshPlugin (DefaultPlugins) always provides.

BSN authoring (crates/buiy_bsn + widgets)

• buiy_bsn: thin prelude re-exporting bsn!/bsn_list! + spawn ext traits; surfaced via buiy::bsn / buiy::prelude. Widget-agnostic.
• Node now #[require]s the style decomposition (sync_styles already queried it non-optionally — mandatory companions; fixes latent bare-Node fragility). Widgets require Node + companions, so bsn!{ Button } materializes the full contract; existing constructors unchanged.
• Widget scene-fns (button/text_input_*) spell styling as field-patches so user patches merge field-wise (the #[require] patch-suppression remedy) — one source of truth with the require initializers.
• Closed the 8 reflect-registration gaps from the BSN-readiness audit.
• examples/hello_bsn: a real Buiy UI authored via bsn!, gated by a Tier-1 layout snapshot (the Save/Cancel width-vs-padding merge proven in the resolved layout).

Verification

• fmt / clippy -D / doc -D / cargo deny: ✅
• Headless suites: ✅ across all crates (buiy_core 994, widgets, bsn, meta, verify 170, hello_bsn, workspace).
• buiy_core GPU --ignored lane (render-rewrite correctness — the dev gate): ✅ on a real adapter (render_compositor_gpu, golden, msaa, atlas). Button reftest: ✅.
• Caveat (not a regression): the one zero-tolerance buiy_verify lavapipe golden (rect-rounded/golden_sdf_corner) diverges by max_channel_delta=35 / 0 differing pixels / mssim 0.9995 — a sub-pixel AA delta from the dev-host Mesa 26.0.2 vs CI-pinned Mesa 24.3.4 (the baselines are Mesa-pinned for CI; also naga 27→29). Documented in follow-ups.md for CI re-bless on the pinned Mesa; not blessed on a dev host (would break CI).

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