Expand the board to the full sketch (~107 spaces, three forks)

shared/board.js now transcribes every distinct space label from the
hand-drawn sketch (assets/game_board.png) rather than Phase 1's small
subset. The sketch's arrows get genuinely ambiguous in a few places (it
reads as a mockup, not an engineered spec) and reuses several space names
across zones (Start A Business, Family Reunion, Market Crash, ...) — that
repetition is kept as intentional flavor rather than deduplicated, and the
ambiguous bits are resolved into a clean DAG with the same shape Phase 1
proved out: a fork, a chain() per branch, a convergence — repeated three
times (Career/Education/Gap Year, then Relationship/Investment, then High
Risk/Safe), into a long shared retirement tail. Branches are built with a
small chain() helper that auto-wires each entry's `next` to the following
one, since hand-wiring ~107 ids was too error-prone.

The reducer, SQLite schema, and rooms/WS layer needed zero changes — the
whole point of the pure-reducer/graph-data design from Phase 1. Two things
did need generalizing:
- public/boardRender.js's lane offset was hardcoded to a 2-way fork; the
  new "Which Path?" fork is 3-way (Career/Education/Gap Year), so the
  offset formula is now symmetric for any number of branches.
- shared/game.test.js hardcoded Phase 1's specific space ids. Rewritten to
  be board-structure-agnostic: it always resolves the first offered choice
  and otherwise rolls the max die value, which reliably makes progress
  regardless of board shape (walkForward always stops early at the next
  choice/finish), plus a graph-well-formedness check.

Verified: unit tests green; a full two-player game played headlessly
end-to-end through all three forks to Finish in 38 turns with zero
console/page errors; the rendered board visually confirmed at full scale
(5-row snake layout, 3-way fork fans out correctly, all labels legible).

public/index.html

@@ -70,7 +70,7 @@
 
   /* --- Board --- */
   .board-container {
-    overflow-x: auto; border-radius: 14px; margin-bottom: 14px;
+    overflow: auto; max-height: 620px; border-radius: 14px; margin-bottom: 14px;
     background: radial-gradient(900px 500px at 30% 0%, #2a5f4c 0%, #1d493c 55%, #163a30 100%);
     border: 1px solid var(--line);
   }