feat: add projection grammar

[pesap]

Summary

This PR adds and hardens projection-based reduce support for nodal allocation, and aligns compiler behavior, tests, and docs.

What is new

• Added top-level projection parsing/AST support and semantic validation.
• Added reduce-projection expression lowering (e.g. reduce "ai" { sum "investment" }).
• Extended tuple-domain reduction handling so projected reductions bind correctly to tuple rows.
• Updated nodal-allocation example and related CLI/KDL test expectations.
• Tightened parser behavior for reduce-projection detection (anchored parse).
• Updated docs/arco-spec.md and examples/README.md to match implemented behavior.

New KDL example (expression used in a constraint)

data "links" source="data/links.csv" {
  set "area" as="a"
  set "tech" as="i"
  alias "generators" column="gen"
  alias "buses" column="bus"

  set "generators" as="g"
  set "buses" as="b"

  set "feasible_links" {
    index "a" { in "area" }
    index "i" { in "tech" }
    index "g" { in "generators" }
    index "b" { in "buses" }
    filter { feasible > 0 }
  }

  param "mw_target" {
    index "a"
    index "i"
    reduce "max"
  }
}

set "feasible_ai" {
  in "feasible_links"
  index "a" { in "area" }
  index "i" { in "tech" }
}

projection "ai" {
  from "feasible_links"
  to "a" "i"
}

model "NodalAllocation" {
  param "mw_target" {
    index "a"
    index "i"
  }

  control "investment" lower=0 {
    index "a" { in "feasible_links" }
    index "i" { in "feasible_links" }
    index "g" { in "feasible_links" }
    index "b" { in "feasible_links" }
  }

  expression "investment_by_area_tech" {
    reduce "ai" {
      sum "investment"
    }
  }

  constraint "enforce_mw_target" {
    index "a" { in "feasible_ai" }
    index "i" { in "feasible_ai" }
    expression {
      investment_by_area_tech[a,i] >= mw_target[a,i]
    }
  }

  minimize "obj" { 0 }
}

scenario "S1" {
  use "NodalAllocation"
}

Test strategy

• just ci ✅