docs: canon log schema + origin seed design

Design spec for the §11 canon log, plus origin seeds as thin initial-state
overlays over a single static world. Three layers (world content / origin seed /
runtime canon log), the in/out boundary (numeric Luck never enters the AI
payload, §7), field-level schemas, new-game construction order, turn-to-turn
maintenance, and JSON storage validated on both client and api.

Folds content/npcs and content/quests under content/world/; adds
content/world/{locations,items} and content/origins. fallback/ kept outside
world/ (authored prose, not id-referenced).

Spec: docs/superpowers/specs/2026-07-09-canon-log-schema-design.md

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

content/world/README.md

@@ -0,0 +1,16 @@
+# /content/world — static, ID-referenced content
+
+The shared world. Authored once, identical every playthrough. Everything here
+carries a stable string **id** that origins and the canon log reference. Never
+seed initial *state* here — that is an origin's job (`/content/origins`).
+
+```
+/locations   Towns, dungeons, points of interest. id → location
+/npcs        Persona + knowledge lists (charter §6). id → npc
+/quests      Story skeletons and quest definitions (charter §17). id → quest
+/items       Gear, consumables, cursed items (charter §7). id → item
+```
+
+New-game construction resolves every id an origin references (start location,
+start quest, granted items, seeded companions) against this content, and fails
+loudly if one is missing. Keep ids stable — a rename is a breaking change.