docs: 修正 BINARY_CACHE 注释（实际无 TTL/上限、weigher 未生效）

[wnzzer]

Summary

• asset.rs 里 BINARY_CACHE 原注释写"weighted size-based eviction (still useful)"，与实现不符：builder 设了 weigher 但没有 max_capacity，moka 只有在配了 max_capacity 时才按权重驱逐 → 当前永不过期、永不驱逐，weigher 形同虚设。
• 仅改注释，准确描述：无 TTL / 无上限 / 进程内常驻，并说明如何激活容量上限。无行为变更。

背景

排查"图标偶发不显示"时确认：图标闪烁仅来自冷启动缓存填充窗口，与缓存淘汰无关（因为根本不淘汰）。顺手把误导性注释修正。

Test plan

• cargo fmt --check 通过
• cargo clippy --all-targets --all-features -Dwarnings 通过
• 注释级改动，无逻辑变化

[Copilot]

Pull request overview

This PR corrects misleading documentation around the Tauri asset binary cache so the comment matches the current moka cache configuration and runtime behavior.

Changes:

• Clarifies that BINARY_CACHE has no TTL or capacity limit.
• Notes that the configured weigher is inactive unless max_capacity is added.
• Documents the tradeoff of enabling capacity-based eviction later.

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