catalog, site: settle five singular/plural subcategories; V13 folds plurals

Where both forms of a subcategory were in use, the more common one
now stands, on 15 entries:
- character (7) -> characters (9)
- environments (4) -> environment (32)
- interiors (1) -> interior (7)
- tilesets (2) -> tileset (8)
- vector (1) -> vectors (2)

V13 now also treats a trailing "s" as the same value, so the pairs
cannot come back. Before switching it on, plural folding was run
across every formats and subcategories value: its only hits were these
five pairs. One new fixture (52 assertions), and a mutation test on a
real entry (kaykit-character-animations back to "character") fails the
validator as expected. CONTRIBUTING records the kept forms.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Wc67NX9MM964uaVGn4bpZJ

Author: TMHSDigital

CONTRIBUTING.md

@@ -43,11 +43,11 @@ One new markdown file is enough — the website rebuilds from frontmatter on dep
 - `attribution_required`: `true` / `false` / `unknown`. If the licence normally requires credit (CC-BY) but the publisher waives it, set `false` and add a `- Attribution waived:` line in Notes saying so.
 - `attribution_string`: optional copy-paste credit.
 - `publisher`: optional. Name the **rights-holding publisher**, never the host. Set it whenever that publisher has more than one catalog entry, so the entries group; setting it on a publisher that currently has only one entry is also fine and saves a backfill later. Do not set it to a generic host (GitHub, Hugging Face, itch.io) or to a distributor that does not hold the rights. Distinguish sibling organisations that are genuinely different rights holders: `blender.md` (the application, Blender Foundation) carries no `publisher`, while the asset bundles under `studio.blender.org` carry `Blender Studio`. Kenney, Quaternius, KayKit, Google Fonts, OpenGameArt, LuizMelo, 0x72, Blender Studio, Material Maker, Alif Type, GGBotNet and 3dmodelscc0 are the largest groups in use today; treat that as illustrative, not as the permitted set.
-- `subcategories`: lower-case kebab-case (`base-meshes`, `field-recordings`). Reuse a value already in the catalog before inventing one, and do not add a variant of an existing value that differs only in plural or spelling.
+- `subcategories`: lower-case kebab-case (`base-meshes`, `field-recordings`). Reuse a value already in the catalog before inventing one, and do not add a variant of an existing value that differs only in plural or spelling. Where both forms were in use, the more common one was kept (`characters`, `environment`, `interior`, `tileset`, `vectors`).
 - `formats`: file formats and engine targets in their usual spelling: extensions upper case (`PNG`, `FBX`, `JSON`, `VOX`), product names as the product writes them (`glTF`, `Godot`), and lower-case kebab-case for package types (`godot-addon`, `blender-extension`). Reuse the spelling already in the catalog.
 - `grid_dimensions` / `camera_perspective` / `hardware_tags`: optional 2D/UI metadata.
 - `verified`: ISO date (`YYYY-MM-DD`) of your last license check.
-- `status`: `active` | `needs-review` | `deprecated`. `active` means the licence, the commercial stance and the credit requirement are all settled: an `unknown` in `license`, `commercial` or `attribution_required` keeps an entry at `needs-review`. The validator enforces this (V14), and rejects `formats` or `subcategories` values that differ only by case or punctuation from one already in use (V13).
+- `status`: `active` | `needs-review` | `deprecated`. `active` means the licence, the commercial stance and the credit requirement are all settled: an `unknown` in `license`, `commercial` or `attribution_required` keeps an entry at `needs-review`. The validator enforces this (V14), and rejects `formats` or `subcategories` values that differ only by case, punctuation or a trailing "s" from one already in use (V13).
 
 ## License verification checklist
 

catalog/2d/craftpix.md

@@ -3,7 +3,7 @@ id: craftpix
 name: CraftPix (freebies)
 url: https://craftpix.net/freebies/
 category: 2d
-subcategories: [ui, sprites, tilesets]
+subcategories: [ui, sprites, tileset]
 license: custom
 commercial: true
 attribution_required: false

catalog/2d/glitch-archive.md

@@ -4,7 +4,7 @@ name: Glitch (CC0 asset release)
 url: https://github.com/tinyspeck/glitch-items
 publisher: Tiny Speck
 category: 2d
-subcategories: [sprites, vectors, environments]
+subcategories: [sprites, vectors, environment]
 license: CC0
 license_spdx: CC0-1.0
 commercial: true

catalog/2d/kenney-roguelike-characters.md

@@ -4,7 +4,7 @@ name: Kenney Roguelike Characters
 url: https://kenney.nl/assets/roguelike-characters
 publisher: Kenney
 category: 2d
-subcategories: [sprites, pixel, character]
+subcategories: [sprites, pixel, characters]
 license: CC0
 license_spdx: CC0-1.0
 commercial: true

catalog/2d/luizmelo-evil-wizard.md

@@ -4,7 +4,7 @@ name: LuizMelo Evil Wizard
 url: https://luizmelo.itch.io/evil-wizard
 publisher: LuizMelo
 category: 2d
-subcategories: [2d, sprites, character]
+subcategories: [2d, sprites, characters]
 license: CC0
 license_spdx: CC0-1.0
 commercial: true

catalog/2d/luizmelo-martial-hero.md

@@ -4,7 +4,7 @@ name: LuizMelo Martial Hero
 url: https://luizmelo.itch.io/martial-hero
 publisher: LuizMelo
 category: 2d
-subcategories: [2d, sprites, character]
+subcategories: [2d, sprites, characters]
 license: CC0
 license_spdx: CC0-1.0
 commercial: true

catalog/2d/luizmelo-monsters-creatures-fantasy.md

@@ -4,7 +4,7 @@ name: LuizMelo Monsters Creatures Fantasy
 url: https://luizmelo.itch.io/monsters-creatures-fantasy
 publisher: LuizMelo
 category: 2d
-subcategories: [2d, sprites, character]
+subcategories: [2d, sprites, characters]
 license: CC0
 license_spdx: CC0-1.0
 commercial: true

catalog/2d/opengameart.md

@@ -4,7 +4,7 @@ name: OpenGameArt
 url: https://opengameart.org
 publisher: OpenGameArt
 category: 2d
-subcategories: [sprites, tilesets, aggregator]
+subcategories: [sprites, tileset, aggregator]
 license: varies
 commercial: unknown
 attribution_required: unknown

catalog/3d/3dmodelscc0-city.md

@@ -4,7 +4,7 @@ name: 3dmodelscc0 City Environment Pack
 url: https://3dmodelscc0.itch.io/city-environment-pack
 publisher: 3dmodelscc0
 category: 3d
-subcategories: [props, interiors, environment]
+subcategories: [props, interior, environment]
 license: CC0
 license_spdx: CC0-1.0
 commercial: true

catalog/3d/kaykit.md

@@ -4,7 +4,7 @@ name: KayKit (Kay Lousberg)
 url: https://kaylousberg.itch.io
 publisher: KayKit
 category: 3d
-subcategories: [characters, environments, animation]
+subcategories: [characters, environment, animation]
 license: CC0
 license_spdx: CC0-1.0
 commercial: true