Initial spec for Sagaphone

Pulled from Notion project docs: overview, architecture, data model,
event system, line logic, and implementation notes for the family-tree
generator + TTS narrator.

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

spec/line-logic.md

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+# Line Logic
+
+- A **family line** is a branch: one person and all of their descendants.
+- All descendants count — legitimate, illegitimate, children from multiple
+  partnerships.
+- The active line stays on the current branch until it dies out.
+- "Dies out" means nobody in that branch has living descendants (checked up
+  to `current_generation - 1`).
+- When a branch dies out → a random pick among the other existing lines.
+- Background persons (not currently narrated) are still simulated — events,
+  death, partnerships, children — just not narrated.
+
+## Edge Cases
+
+- At least one line must always remain.
+- If the last line dies out prematurely → search earlier generations for
+  branches that haven't been narrated yet.
+- From generation 6 onwards, no new generations are simulated in the
+  background.