Remove legacy unified-format aliases (target: v2.0.0 major)

[apotema]

Goal

Drop the deprecated legacy paths that were kept as a compatibility bridge for the unified scene/prefab format (RFC #560). Today every legacy site warns once on first hit; this ticket tracks the removal.

Sites to delete

labelle-engine

src/jsonc/unified_format.zig:

• getObject("entities") legacy top-level array fallback — currently warns: "legacy 'entities' key: wrap the entity array in a 'root' block and rename it to 'children'"
• getObject("components") fallback on prefab references — currently warns: "legacy 'components' on a prefab reference: rename it to 'overrides'"
• Top-level "assets" key — currently ignored with warning: "legacy 'assets' key is ignored — assets are inferred from sprite references"

src/jsonc/scene_loader.zig:

• The "legacy top-level entities array" branch at line 392
• Position-component-on-legacy-prefabs path at line 454

src/jsonc/unified_format.zig warn-once infrastructure (the global dedup map keyed on comptime literals — can go once nothing warns).

labelle-assembler

src/config.zig:

• The initial_scene: ?[]const u8 deprecated field
• normalizeInitialPrefab() deprecation alias logic + the stderr deprecation notice

src/codegen/scan.zig:

• The legacy scene-format scan path (matches the engine's unified loader)

labelle-cli

src/cli/project_config.zig:

• The initial_scene: ?[]const u8 deprecated field (mirror of assembler)
• normalizeInitialPrefab() helper

src/cli.zig:

• The parsed.initial_prefab = scene rewrite at the cli#229 site that's flagged as a "deprecation bridge" — once LABELLE_SCENE is the only path, this can go too.

Pre-removal verification

Before deleting, run labelle audit unification (cli #232, shipped in v1.42.0) across:

• bouncing-ball
• flying-platform-labelle
• Every project in labelle-assembler/examples/
• Every project in labelle-cli/test-projects/

…with the deprecation warnings promoted to errors (a -Dstrict=true build flag would work, or a temporary @compileError in the legacy branches). Any project that still trips a warning gets migrated first. The audit doesn't check for legacy "entities" / "components" patterns yet — extending it is a small follow-up (filed as part of this work) or a precursor PR.

Timing

Slated for the v2.0.0 major engine bump. The current v1.x train keeps legacy support. Open question for whoever picks this up: should v2.0 also bundle other breaking-API cleanups (e.g. engine.requestedScreenshot vs engine.ScreenshotRequest.parse, or other surface that has been silently renamed) to amortize the major?

Out of scope (do not touch in this PR)

• The pathToIdent underscore-escape behavior (separate ticket Remove zgpu backend support #212).
• The compat-checker false positive for 0.x plugins (cli#230).
• §B2 engine-side enforcement (loader: enforce RFC #560 §B2 ({prefab + children} rejection) directly in the scene loader #586) — orthogonal but recommended to land first so the unified loader actually rejects what the RFC says it should.

Related

• RFC RFC: unify scenes and prefabs #560 (closed) — the unification design
• RFC §unification: loader accepts unified prefab/scene format #561 — loader-accepts-unified tracker (still open)
• loader: enforce RFC #560 §B2 ({prefab + children} rejection) directly in the scene loader #586 — §B2 engine-side enforcement (recommended prerequisite)
• cli trackEntity API changed: second parameter now expects VisualType instead of Registry #232 — labelle audit unification command

[apotema]

Update: bundling root-removal into v2.0

Adding phase 2 to this epic: drop the `root:` wrapper from unified-format files. Same migration cycle, same audit infrastructure, same loader-path-deletion at v2.0 — folding it in costs almost nothing extra design-wise.

Design RFC opened: RFC-FLATTEN-ROOT.md.

Revised v2.0 scope

Phase	Removal
1 (this ticket's original scope)	Legacy `entities` key
1	Legacy `components` on prefab refs
1	Legacy `assets` array
1	Legacy `.initial_scene` field
2 (new — RFC #594)	root: wrapper itself

Sequence (revised)

1. v1.47.0 (or next minor): loader accepts flat form, audit detects `legacy_root_wrapper` and the other four legacy patterns. Migrator (`labelle migrate unified`) ships.
2. v1.47.x: projects clean up via the migrator at their own pace.
3. v2.0.0: loader drops all five legacy paths. Single breaking bump.

Why bundle vs stage

• Same migration tooling has to be built anyway — `labelle migrate unified` handles all five transforms.
• Same audit walker — one additional finding type.
• Two close-together major bumps would fatigue users; one cleaner bump is better.
• The migrator removes the migration friction either way.

Discussion of timing knobs (v1.47 deprecation warning yes/no, generator switch timing, etc.) is on the RFC PR.