B6 last card rebalance: speed, density, frenzy redesign

[fldlqu]

B-6 boss 最后一张卡「炫妈归天」全面平衡

• 弹速全部 ×0.5
• 弹幕密度阶梯下调：初始 -> -20% -> -30%
• 狂乱阶段重做：aim 改为札弹（n:5→2, speed/1.45），新增左右侧翼固定扇形札弹源（各 n:2），清除旧 storm/ring/spin/rain
• letterTime 60→120s，新增超时强制开狂乱（60s HP>40% 强制、90s 兜底）
• storm 散射改为 360° 6 发环弹

See commit d38bdad for full diff.

本 PR 旨在重新平衡 B-6 Boss 最终卡“炫妈归天”的弹幕难度与狂乱阶段节奏：整体降低子弹速度和密度，重构狂乱弹幕为札弹及固定侧翼发射，移除原有风暴、环形、旋转和雨弹模式，并调整风暴为 360° 六弹环。与此同时延长卡牌时限，增加基于时间与血量的狂乱触发机制，并将版本号更新至 128。

[coderabbitai]

📝 Walkthrough

Walkthrough

Changes

B6 Boss 与发布信息

Layer / File(s)	Summary
B6 Boss 狂乱弹幕与状态流转 js/stages/b6/letters.js	调整 storm、aim、ring 参数，新增超时强制狂乱逻辑，并以左右 wing_l/wing_r 札弹替换狂乱阶段的 spin/rain。
章节时长与构建号更新 js/stages/b6/index.js, js/version.js	章节 id 128 的 letterTime 从 60 改为 120，构建号从 127 改为 128。

Estimated code review effort: 3 (Moderate) | ~20 minutes

Sequence Diagram(s)

sequenceDiagram
  participant BossState
  participant FrenzyTimers
  participant spawnAimed
  BossState->>BossState: 检查 en.age 与 hpRatio
  BossState->>BossState: 设置 en.hp = en.maxHp * 0.19
  BossState->>FrenzyTimers: 进入 frenzy
  FrenzyTimers->>spawnAimed: 由 wing_l/wing_r 生成固定方向札弹

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Suggested reviewers: octobersama

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	标题准确概括了 B6 最终卡的主要改动：速度、密度与狂乱机制重做。
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@js/stages/b6/letters.js`:
- Line 207: 修正 letters.js 中 type 配置的随机索引范围：将 Math.random() 的乘数改为与数组长度一致，使索引可覆盖
0–4，并确保 'large' 类型能够被随机选中。

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Fix all unresolved CodeRabbit comments on this PR:

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• Create a new PR with the fixes

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ℹ️ Review info

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Review profile: CHILL

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Run ID: 0453c4f9-d875-443f-bd6f-1cb010bb20d7

📥 Commits

Reviewing files that changed from the base of the PR and between e730e98 and d38bdad.

📒 Files selected for processing (3)

• js/stages/b6/index.js
• js/stages/b6/letters.js
• js/version.js

[coderabbitai]

🎯 Functional Correctness | 🟡 Minor | ⚡ Quick win

修正随机索引范围。

Line 207 的索引只能取 0–3，数组中的 'large' 永远不会出现。应使用 5 或数组长度，避免弹型配置失效。

建议修复

-          type: ['dot', 'rice', 'talisman', 'medium', 'large'][Math.floor(Math.random() * 4)],
+          type: ['dot', 'rice', 'talisman', 'medium', 'large'][Math.floor(Math.random() * 5)],

📝 Committable suggestion

‼️ IMPORTANT
Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation. Thoroughly test & benchmark the code to ensure it meets the requirements.

Suggested change

	type: ['dot', 'rice', 'talisman', 'medium', 'large'][Math.floor(Math.random() * 4)],
	type: ['dot', 'rice', 'talisman', 'medium', 'large'][Math.floor(Math.random() * 5)],

🤖 Prompt for AI Agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@js/stages/b6/letters.js` at line 207, 修正 letters.js 中 type 配置的随机索引范围：将
Math.random() 的乘数改为与数组长度一致，使索引可覆盖 0–4，并确保 'large' 类型能够被随机选中。