Effect v4

Routing

• Do not read this file linearly. Pick one target.
• Match the task to a folder below.
• For API lookup, open the file whose title fits.
• For implementation or review, read that folder's index.md and the relevant examples. The index contains behavioral rules that examples may not repeat.
• Within a folder, the lowest-numbered file is the lead example for that topic; higher numbers add variations.
• fixtures/ folders hold supporting modules for an example, not standalone topics.
• Migrating existing v3 code? Go to Migration first.

Identify the version