DIFF-466：access.json strict schema + 产品窗证据 README

变更

• VaultMcpAccess 解码改用 ignoreUnknownKeys = false，对齐 Electron accessSchema strictObject；未知字段抛 IllegalArgumentException（包装为既有「Cannot read…」文案）。
• VaultMcpAccessTest.readRejectsUnknownJsonProperties。
• scripts/prepare-git-merge-conflict-evidence.sh：init.defaultBranch=main、git init -b main。
• scripts/README-product-evidence.md：§A/§B/IME 脚本用法与「勿 eval 带注释输出」说明。

验证

./gradlew :composeApp:desktopTest --offline --tests 'com.rememberber.mootool.next.compose.ai.VaultMcpAccessTest'
./gradlew :composeApp:desktopTest --offline

产品窗 PNG 仍须人工：docs/evidence/2026-09-17-vault-conflict-product-window/results.md。