feat(builder): schema-driven Build page for world objects (#11)

Select a Part or Model in Studio, answer "what is this object?", fill a form —
it becomes a gatherable node, a mob or a quest giver. Closes the gap between
authoring a def and setting up a world object, which until now meant knowing to
tag a part and hand-typing PascalCase attributes in the property panel.

Engine — components can declare an attribute SCHEMA:
- src/components/Schema.luau (new): AttributeSpec/Display/ComponentSchema types,
  normalize/defaults/get/list, and the schemas for Gatherable, Mob, QuestGiver.
  Dependency-free ON PURPOSE: the plugin requires it live at edit time, and the
  component modules themselves can't be required there (Harvesting asserts
  IsServer; Remotes creates instances in ReplicatedStorage).
- Components.define now accepts EITHER the legacy `attr = default` map or a
  schema array, normalizing both to one ordered spec list; bind() reads the
  derived default map, so binding is byte-identical. Legacy maps are sorted, as
  `pairs` order is arbitrary and would make a UI jitter. New getSchema/
  listSchemas. The three shipped components pull name/tag/display/attributes
  from the schema; their onSetup bodies are untouched (defaults verified
  identical, all 23 attributes).

Plugin — the Build page:
- Field.luau (new): coerce/format/equalsDefault, lifted from ConfigAdmin (which
  now delegates), shared by every schema-driven editor.
- FieldRow.luau (new): the shared [○/●] label … control + help row, including a
  ⌄ picker that cycles authored ids for fields declaring `ref`.
- BuildAdmin.luau (new): live schema read with three distinct empty states,
  selection/eligibility/identify, deltas-only attribute writes, applyType
  (tag + clear any other component) and clear.
- BuildAdminUi.luau (new): chooser cards, grouped form, multi-select apply,
  stale-bind-marker warning, SelectionChanged-driven refresh.
- init.server.luau: record()-wrapped buildActions + the page.

Docs: docs/admin-plugin.md Build section + a 60-second walkthrough,
docs/extending.md schema guide, a CONTRIBUTING rule that new creator components
declare one, CHANGELOG.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

sourcemap-plugin.json

@@ -1 +1 @@
-{"name":"SurvivorCoreStatAdmin","className":"Script","filePaths":["plugin/init.server.luau","plugin.project.json"],"children":[{"name":"ConfigAdmin","className":"ModuleScript","filePaths":["plugin/ConfigAdmin.luau"]},{"name":"ConfigAdminUi","className":"ModuleScript","filePaths":["plugin/ConfigAdminUi.luau"]},{"name":"ContentAdmin","className":"ModuleScript","filePaths":["plugin/ContentAdmin.luau"]},{"name":"ContentAdminUi","className":"ModuleScript","filePaths":["plugin/ContentAdminUi.luau"]},{"name":"HudPreview","className":"ModuleScript","filePaths":["plugin/HudPreview.luau"]},{"name":"Nav","className":"ModuleScript","filePaths":["plugin/Nav.luau"]},{"name":"OverviewPage","className":"ModuleScript","filePaths":["plugin/OverviewPage.luau"]},{"name":"SearchPage","className":"ModuleScript","filePaths":["plugin/SearchPage.luau"]},{"name":"SpawnHelpers","className":"ModuleScript","filePaths":["plugin/SpawnHelpers.luau"]},{"name":"StatAdmin","className":"ModuleScript","filePaths":["plugin/StatAdmin.luau"]},{"name":"StatAdminUi","className":"ModuleScript","filePaths":["plugin/StatAdminUi.luau"]},{"name":"Theme","className":"ModuleScript","filePaths":["plugin/Theme.luau"]}]}
+{"name":"SurvivorCoreStatAdmin","className":"Script","filePaths":["plugin/init.server.luau","plugin.project.json"],"children":[{"name":"BuildAdmin","className":"ModuleScript","filePaths":["plugin/BuildAdmin.luau"]},{"name":"BuildAdminUi","className":"ModuleScript","filePaths":["plugin/BuildAdminUi.luau"]},{"name":"ConfigAdmin","className":"ModuleScript","filePaths":["plugin/ConfigAdmin.luau"]},{"name":"ConfigAdminUi","className":"ModuleScript","filePaths":["plugin/ConfigAdminUi.luau"]},{"name":"ContentAdmin","className":"ModuleScript","filePaths":["plugin/ContentAdmin.luau"]},{"name":"ContentAdminUi","className":"ModuleScript","filePaths":["plugin/ContentAdminUi.luau"]},{"name":"Field","className":"ModuleScript","filePaths":["plugin/Field.luau"]},{"name":"FieldRow","className":"ModuleScript","filePaths":["plugin/FieldRow.luau"]},{"name":"HudPreview","className":"ModuleScript","filePaths":["plugin/HudPreview.luau"]},{"name":"Nav","className":"ModuleScript","filePaths":["plugin/Nav.luau"]},{"name":"OverviewPage","className":"ModuleScript","filePaths":["plugin/OverviewPage.luau"]},{"name":"SearchPage","className":"ModuleScript","filePaths":["plugin/SearchPage.luau"]},{"name":"SpawnHelpers","className":"ModuleScript","filePaths":["plugin/SpawnHelpers.luau"]},{"name":"StatAdmin","className":"ModuleScript","filePaths":["plugin/StatAdmin.luau"]},{"name":"StatAdminUi","className":"ModuleScript","filePaths":["plugin/StatAdminUi.luau"]},{"name":"Theme","className":"ModuleScript","filePaths":["plugin/Theme.luau"]}]}