Function updateTimelineField

docs/assets/script.js:4441–4549

* 核心：修改 timeline YAML 中的指定字段，保留注释

(presetName, periodKey, field, value)

 * 核心：修改 timeline YAML 中的指定字段，保留注释
 */
function updateTimelineField(presetName, periodKey, field, value) {
    const editor = document.getElementById('timeline-editor');
    let yaml = editor.value;
    const lines = yaml.split('\n');

    // 1. 定位预设/custom 的起始行
    const isCustom = presetName === 'custom';
    let sectionStart = -1;
    let sectionIndent = 0;

    if (isCustom) {
        // 找 custom: 顶层 key
        for (let i = 0; i < lines.length; i++) {
            if (/^custom:\s*/.test(lines[i])) {
                sectionStart = i;
                sectionIndent = 0;
                break;
            }
        }
    } else {
        // 找 presets: 下的 presetName:
        let inPresets = false;
        for (let i = 0; i < lines.length; i++) {
            const line = lines[i];
            if (/^presets:\s*/.test(line)) {
                inPresets = true;
                continue;
            }
            if (inPresets && /^\S/.test(line) && !line.startsWith('#')) {
                break; // left presets block
            }
            if (inPresets) {
                const m = line.match(/^(\s+)(\S+):\s*/);
                if (m && m[2] === presetName) {
                    sectionStart = i;
                    sectionIndent = m[1].length;
                    break;
                }
            }
        }
    }

    if (sectionStart < 0) return;

    // 2. 找到 section 结束行
    let sectionEnd = lines.length;
    for (let i = sectionStart + 1; i < lines.length; i++) {
        const line = lines[i];
        if (line.trim() === '' || line.trim().startsWith('#')) continue;
        const indent = line.search(/\S/);
        if (indent <= sectionIndent) {
            sectionEnd = i;
            break;
        }
    }

    // 3. 在 section 内定位 periodKey 子区域
    let targetStart, targetEnd;