feat(kotlin): Implement complete metrics support for Kotlin language

[marlon-costa-dc]

Summary

Implements all 11 metric modules for Kotlin, replacing the stub implementations generated by implement_metric_trait!.

Depends on #1225 (tree-sitter 0.26.5 update — provides the tree-sitter-kotlin-codanna grammar).

Metrics implemented

Metric	Description
ABC	Assignment-Branch-Condition with Kotlin-specific helpers
Cognitive	Nesting-aware complexity (if, when, for, while, do, catch, &&, ||)
Cyclomatic	Control flow complexity
Exit	Return statement detection
Halstead	Full operator/operand classification for Kotlin tokens
LOC	Lines of code
NArgs	Number of arguments
NOM	Number of methods
NPA	Number of public attributes (via modifier list traversal)
NPM	Number of public methods (via modifier list traversal)
WMC	Weighted method complexity

Key implementation details

• Checker trait (checker.rs): Maps tree-sitter-kotlin-codanna node types to is_comment, is_func_space, is_func, is_closure, is_string, is_call, is_non_arg
• Getter trait (getter.rs): Full Halstead classification — 34 operators (including ?., ?:, !!, as?, !is, !in, ..<) and all operand types
• Cognitive complexity: Follows Java pattern with nesting penalties and else if chain handling
• NPA/NPM: Uses traverse_children to navigate property_declaration → modifiers → visibility_modifier paths
• WMC: Extracted shared compute_wmc helper for cross-language reuse

Files changed

• src/checker.rs — KotlinCode Checker implementation
• src/getter.rs — KotlinCode Getter with Halstead classification
• src/metrics/abc.rs — ABC metric with 3 Kotlin helper functions
• src/metrics/cognitive.rs — Cognitive complexity
• src/metrics/cyclomatic.rs — Cyclomatic complexity
• src/metrics/exit.rs — Exit/return detection
• src/metrics/halstead.rs — Halstead metric
• src/metrics/loc.rs — Lines of code
• src/metrics/nargs.rs — Number of arguments
• src/metrics/npa.rs — Number of public attributes
• src/metrics/npm.rs — Number of public methods
• src/metrics/wmc.rs — Weighted method complexity
• Also includes node.rs + asttools.rs extensions needed by metrics

Replaces

Replaces #1215 (closed due to messy branch history). Clean version rebased on feature/tree-sitter-0.26.5.

Test Plan

• cargo check passes
• All 252 existing tests pass (cargo test --workspace)
• 2 clean commits (tree-sitter update base + kotlin metrics)