feat(content): calling + race blurbs, loaded by ContentDB

Hand-written like items/ — no build-tool namespace needed. Blurbs are content;
mechanics are code, and a test asserts the blurb files carry no hit dice. The
prose is placeholder; swapping it later touches no code, which is what stops the
content BOM from blocking the engine.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QYa9u7Kdxv5gX4AnwWexy8

content/world/races/elf.json

@@ -0,0 +1 @@
+{ "id": "elf", "name": "Elf", "blurb": "You see in the dark and you notice things. Neither has made you popular in a town that would rather not be noticed." }