Boot-Time Environment Validation

Validate environment variables when the process boots. Fail with a readable message. Do not scatter process.env through the app. APP Runtime: [NODE | EDGE] Required in production: [LIST] Required only for some features: [LIST] Public vars that are allowed in the browser: [LIST] Secrets: [LIST] DELIVER 1) A schema (Zod or the equivalent I use) that parses once, coerces numbers, and rejects empty strings. 2) The error format: variable name, what is wrong, no printing of the secret value. 3) The rule for NEXT_PUBLIC_ or other public prefixes: a test that fails if a secret key is given a public name. 4) How tests and CI set the minimum env without a real .env file. 5) Feature flags that are env-driven: a typed getter, not string comparisons at the call site. Show the one import path the rest of the code uses. Direct process.env outside that module is a review comment. Say that.