feat(kdl): indexed bounds syntax for var blocks

[pesap]

Summary

Add support for indexed bounds in var blocks using lower { expr } and upper { expr } child nodes, allowing bounds to reference parameters.

Parent: #176
Blocked by: #179

Syntax

// Scalar bounds (existing, inline)
var x lower=0 upper=100 {index generators; index buses}

// Indexed bounds (new, child nodes)
var x lower=0 {
    index generators; index buses
    upper { capacity[g] }
}

// Both bounds indexed
var flow {
    index arcs
    lower { min_flow[a] }
    upper { max_flow[a] }
}

Acceptance Criteria

• Parser accepts lower { expr } as child node in var block
• Parser accepts upper { expr } as child node in var block
• Inline scalar bounds (lower=0) still work
• Mixed: inline lower=0 + child upper { expr } works
• Expression inside bounds parsed by math expression parser
• Test: indexed upper bound parses correctly
• Test: indexed lower bound parses correctly
• Test: mixed scalar/indexed bounds work
• Test: expression references parameter correctly

Implementation Notes

Location: crates/arco-kdl/src/source/parser.rs (in parse_var or similar)

The lower { expr } / upper { expr } nodes contain a math expression that gets parsed by the existing expression parser.

• AFK — extends existing var parsing with clear pattern