bug(filter): bare RHS identifiers mis-evaluated as column refs in data filters

[pesap]

Problem

filter { tech == wind } evaluates wind as identifier/column lookup, not categorical literal.
Result: filter resolves false/None and drops all rows.

This conflicts with spec §9:

• Bare RHS identifier (e.g., thermal) should be categorical string value.

Repro

Minimal data:

tech
wind
solar
wind

KDL:

set "idx_wind" {
  in "idx"
  filter { tech == wind }
}

Observed: subset size = 0
Expected: subset size > 0 (rows where tech == "wind")

Scope

• crates/arco-kdl/src/semantic/sets.rs
• crates/arco-kdl/src/compile/data_tables.rs (keep behavior aligned for param filters)

Acceptance criteria

• Bare RHS identifiers in comparisons behave as categorical literals.
• Quoted RHS strings still work.
• Numeric comparisons unchanged.
• Add regression tests for set + param filters.