docs(assets): codify the authored-footprint rule (araminta ruling, PR #214)

footprint_tiles is an authored value, never derived from normalized
mesh geometry — [1,1] defaults only for plausibly-single-tile classes;
ordinarily-multi-tile classes require an explicitly stated footprint at
promotion. Her exact wording in furniture-props.md §3; conventions.md
§4 gains the authored-not-derived sentence pointing at the full rule
(and a pre-existing 'Single Sunday-tile' typo fixed in passing). The
[1,1] values shipped for the four promoted items stand as ruled.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/assets/briefs/furniture-props.md

@@ -76,6 +76,17 @@ manifest for any furniture with a real multi-tile footprint (a table spanning
 `[2, 1]`); most `props`-category items, including `lion_statue`, omit the
 field or set `[1, 1]`.
 
+**Footprint for normalized assets** (araminta ruling, PR #214).
+Blender-normalized GLBs (`target-width=1.0` postprocess, the spike default)
+lose real-world scale — footprint cannot be derived from geometry after
+normalization. `footprint_tiles` is therefore an **authored** value, not a
+measured one: default `[1, 1]` only holds when the object's real-world class
+is plausibly single-tile (chairs, stools, small props, compact desks). Any
+item whose class is ordinarily multi-tile (dining tables seating 4+, couches,
+beds, wardrobes) needs the promoting author to state an intended real-world
+footprint explicitly (in the promotion commit/PR, or a per-item scale note in
+the brief) — never left to infer from the normalized mesh.
+
 ## 4. Generation Path
 
 Two paths, depending on whether the spike asset is being promoted or a new