Add level 8: Scroll of Tags (tags + newlines)

Three keys:
- scroll: multi-line string (block scalar `|`)
- weight: fractional float (e.g. 12.5) — forces float without needing
  the `!!float` tag in the target text
- title:  string of digits (player needs quotes or `!!str` to avoid int)

Per-seed deterministic via ChaCha8Rng XOR'd with 0x..08.

Not wired into levels::registry() yet — integration belongs to a follow-up.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/levels/mod.rs

@@ -17,6 +17,7 @@ pub mod l04_list;
 pub mod l05_dict_list;
 pub mod l06_anchors;
 pub mod l07_complex;
+pub mod l08_tags;
 
 use serde::{Deserialize, Serialize};