docs(architecture): add line pool format specification (#308)

Formal spec at docs/architecture/line-pool-format.md defining YAML
structure for dialogue and monologue content files. Covers tag enums,
4-layer filtering pipeline, prerequisite-to-KG mapping, ID format,
validation rules, and Rust loader interface.

Fixes monologue schema: adds required constraint on relationship
prerequisite target/state fields.

Ref: D-028, D-032, D-035, D-041

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

content/_schema/monologue-pool.schema.json

@@ -38,7 +38,7 @@
           "type": "string",
           "minLength": 1,
           "maxLength": 160,
-          "description": "Line text — 160 char max per D-059"
+          "description": "Line text — 160 char max to fit monologue display without scrolling"
         },
         "trigger": {
           "type": "string",
@@ -63,6 +63,8 @@
             },
             "relationship": {
               "type": "object",
+              "required": ["target", "state"],
+              "additionalProperties": false,
               "properties": {
                 "target": { "type": "string" },
                 "state": {