Function validateEnvAgainstSources

Validate a composed env against config-provided validation surfaces. Precedence for validation definitions: project.local -> project.public -> packaged

Behavior:

• If a JS/TS schema is present, use schema.safeParse(finalEnv).
• Else if requiredKeys is present, check presence (value !== undefined).
• Returns a flat list of issue strings; caller decides warn vs fail.