Implement HC018: Top-level local declaration detection

## Overview
Detects top-level `local` declarations (outside of methods/boxes), which are cleanup omissions in Hakorune code.

## Implementation Details
- **Rule**: `rule_top_level_local.hako` following box principles
- **Detection Method**: Text-based scanning with context tracking
  - Tracks box/method entry/exit via brace depth
  - Identifies `local` statements outside method scope
  - Filters out comments (lines starting with `//`)
- **Integration**: Added to cli.hako in text-based rules section

## Technical Approach
- **Context Tracking**: Maintains `in_box` and `in_method` flags
- **Brace Depth Counter**: Tracks `{` and `}` to determine scope boundaries
- **Line-by-line Analysis**: Checks each line for `local ` prefix when not in method
- **Comment Filtering**: Ignores commented-out local declarations

## Test Cases
- **ok.hako**: All `local` declarations inside methods → no warnings
  - Helper.calculate() and Helper.process() both referenced from Main.main()
  - Avoids HC011 (unreachable method) warnings
- **ng.hako**: Top-level `local global_temp` outside any method → HC018 warning

## Test Results
```
[TEST/OK] HC011_dead_methods
[TEST/OK] HC012_dead_static_box
[TEST/OK] HC013_duplicate_method
[TEST/OK] HC014_missing_entrypoint
[TEST/OK] HC016_unused_alias
[TEST/OK] HC017_non_ascii_quotes
[TEST/OK] HC018_top_level_local ← NEW
[TEST/SUMMARY] all green
```

## Diagnostic Format
```
[HC018] top-level local declaration (not allowed): <path>:<line>
```

## Architecture
- Box-first design: RuleTopLevelLocalBox with single responsibility
- Helper methods: _trim(), _is_comment(), _split_lines(), _itoa()
- Clean separation of concerns: parsing, context tracking, reporting

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

tools/hako_check/cli.hako

@@ -12,6 +12,7 @@ using tools.hako_check.rules.rule_non_ascii_quotes as RuleNonAsciiQuotesBox
 using tools.hako_check.rules.rule_dead_static_box as RuleDeadStaticBoxBox
 using tools.hako_check.rules.rule_duplicate_method as RuleDuplicateMethodBox
 using tools.hako_check.rules.rule_missing_entrypoint as RuleMissingEntrypointBox
+using tools.hako_check.rules.rule_top_level_local as RuleTopLevelLocalBox
 using tools.hako_check.render.graphviz as GraphvizRenderBox
 using tools.hako_parser.parser_core as HakoParserCoreBox
 
@@ -79,6 +80,7 @@ static box HakoAnalyzerBox {
       // HC017 must inspect original text prior to sanitize
       RuleNonAsciiQuotesBox.apply(text_raw, p, out)
       RuleJsonfragUsageBox.apply(text, p, out)
+      RuleTopLevelLocalBox.apply(text, p, out)
       // rules that need IR (enable dead code detection)
       local before_n = out.size()
       RuleDeadMethodsBox.apply_ir(ir, p, out)