📚 docs: Record field declaration design discussion in papers

## Summary
Documented the "init block vs fields-at-top" design discussion as a valuable example of AI-human collaboration in language design.

## Changes

### Paper G (AI Collaboration)
- Added field-declaration-design.md documenting the entire discussion flow
- Showcased how complex init block proposal evolved to simple "fields at top" rule
- Demonstrates AI's tendency toward complexity vs human intuition for simplicity

### Paper H (AI Practical Patterns)
- Added Pattern #17: "Gradual Refinement Pattern" (段階的洗練型)
- Documents the process: Complex AI proposal → Detailed analysis → Human insight → Convergence
- Field declaration design as a typical example

### Paper K (Explosive Incidents)
- Added Incident #046: "init block vs fields-at-top incident"
- Updated total count to 46 incidents
- Shows how a single human comment redirected entire design approach

## Design Decision
After analysis, decided that BoxIndex should remain a compiler-internal structure, not a core Box:
- Core Boxes: User-instantiable runtime values (String, Integer, Array, Map)
- Compiler internals: BoxIndex for name resolution (compile-time only)
- Clear separation of concerns between language features and compiler tools

## Philosophy
This discussion exemplifies key principles:
- The best design needs no explanation
- Constraints provide clarity, not limitation
- "Everything is Box" doesn't mean "compiler internals are Boxes"
- AI tends toward theoretical completeness; humans toward practical simplicity

🐱 Sometimes the simplest answer is right in front of us\!

docs/guides/style-guide.md

@@ -33,6 +33,39 @@ Structure
 - Keep methods short and focused; prefer extracting helpers to maintain clarity.
 - Prefer pure helpers where possible; isolate I/O in specific methods.
 
+Box layout
+- フィールドは box 本体の先頭にまとめる（先頭ブロック）。
+- 先頭フィールド群の後ろはメソッドのみを記述する（`birth` を含む）。
+- フィールド間の空行・コメントは許可。アノテーション（将来追加予定）もフィールド行の直前/行末を許可。
+- NG: 最初のメソッド以降にフィールドを追加すること（リンタ警告／厳格モードでエラー）。
+
+良い例
+```nyash
+box Employee {
+  // データ構造（フィールド）
+  name: StringBox
+  age: IntegerBox
+  department: StringBox
+
+  // ここからメソッド
+  birth(n, a, d) { me.name = n; me.age = a; me.department = d }
+  promote() { }
+}
+```
+
+悪い例（NG）
+```nyash
+box Bad {
+  id: IntegerBox
+  method1() { }
+  name: StringBox  // ❌ フィールドはメソッドの後に置けない
+}
+```
+
+ツール
+- 警告: 既定は警告（`NYASH_CLI_VERBOSE=1` で詳細を表示）。
+- 厳格化: `NYASH_FIELDS_TOP_STRICT=1` でエラーに昇格（Runnerでチェック）。
+
 Examples
 ```nyash
 using core.std as Std
@@ -64,4 +97,3 @@ static box Main {
 CI/Tooling
 - Optional formatter PoC: see `docs/tools/nyfmt/NYFMT_POC_ROADMAP.md`.
 - Keep smoke scripts small and fast; place them under `tools/`.
-