Block spec: inputs/outputs declaration syntax

[pesap]

Description

Define block interface syntax for composable KDL:

block "economic-dispatch" {
    input demand "MW"
    input fuel-cost "$/MMBtu"  
    output generation "MW"
    // internal model...
}

• Type system for inputs/outputs (scalar, vector, time-series)
• Validation that all inputs wired, all outputs produced
• Documentation strings for ports

Acceptance Criteria

• Parser accepts block declarations
• Semantic validation for port types
• Error on missing input wiring

Context

Part of v0.9 (Composable KDL). Foundation for reusable model components.

Target: v0.9