feat: KDL syntax improvements for clarity and consistency

[pesap]

Problem Statement

The current Arco KDL syntax has terminology inconsistencies and ambiguities that confuse users:

1. map keyword — reads as "mapping to a set" but actually renames a CSV column
2. alias overloaded — used for both data column aliases and set shorthand
3. control keyword — optimization users expect var or variable
4. filter keyword — where is more familiar (SQL, Python)
5. Bound syntax — no clear pattern for indexed bounds (e.g., upper=capacity[g])

Solution

Rename keywords for clarity while maintaining backwards compatibility where feasible:

Before	After	Purpose
map X from="col"	alias X column="col"	Data column rename
set X alias=g	set X as=g	Set shorthand
control x lower=0	var x lower=0	Decision variable
filter { cond }	where { cond }	Set subsetting

Additional syntax clarifications:

• Indexed bounds: upper { capacity[g] } / lower { expr } inside var block
• Path resolution: relative to KDL file, overridable via --data-dir or ARCO_DATA_DIR

Implementation Decisions

Parser Changes

• Add alias as new keyword; deprecate map (warn on use, still parse)
• Add as= property on set; deprecate alias= property
• Add var as new keyword; deprecate control (warn on use, still parse)
• Add where as new keyword; deprecate filter
• Parse lower { expr } / upper { expr } as child nodes in var blocks

Path Resolution

• Resolve from="..." paths relative to the input KDL file location
• --data-dir CLI flag overrides (highest priority)
• ARCO_DATA_DIR env var as fallback
• Document resolution order in help/docs

Backwards Compatibility

• control, map, set alias=, filter continue to parse with deprecation warnings
• Deprecation warnings include migration hint (e.g., "use var instead of control")

Testing Decisions

• Unit tests: Parser accepts both old and new syntax; emits correct AST
• Deprecation tests: Old keywords trigger warnings with correct messages
• Path resolution tests: Verify relative, --data-dir, and ARCO_DATA_DIR precedence
• Integration test: Update examples/nodal-allocation/input.kdl to new syntax; verify model runs

Out of Scope

• Multi-objective optimization syntax
• Model linking/composition semantics
• Hierarchical optimization (see ADR-0001)

References

• CONTEXT.md — domain language
• docs/adr/0001-objective-composition-deferred.md — deferred decisions
• examples/nodal-allocation/input.kdl — example to migrate