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Summary

• [code-review] config Save() 双缺口：非 not-exist 读错误被当文件不存在走全量覆盖（#1160 同族漏加固，静默清空未加载字段）+ 外部 section 写失败仅 debug.Log 报成功（#4505 标准未对齐） #4697 (A, high): Save() treated ANY read error as first-write and fell into the full-overwrite branch — deep-merge and tombstone protection skipped, disk-only fields (externally written keys, unknown fields) silently wiped WITH a success report (the temp+rename write only needs directory permissions, so a read failure with a writable dir was the filed trigger). Read errors other than os.IsNotExist now abort, matching the [business-logic] config_save: 损坏的外部配置文件(im.yaml)被 patch 静默清空重写，yaml.Unmarshal 错误被丢弃 #1160 semantics that patchConfigFile/patchExternalFile already apply (the same-file siblings the issue cites).
• [code-review] config Save() 双缺口：非 not-exist 读错误被当文件不存在走全量覆盖（#1160 同族漏加固，静默清空未加载字段）+ 外部 section 写失败仅 debug.Log 报成功（#4505 标准未对齐） #4697 (B, medium): saveExternalSections discarded its three Save* errors — vendors/im/mcp write failures returned Save()=nil, the UI reported success, and the config reverted on restart with no warning. The first failure now propagates through Save().
• [code-review] matrix handleEvent 缺 session 重试在 sync 回调内同步 Sleep 最多 2.4s 且不感知 ctx——连续坏事件串行阻塞整个 sync 事件流水线，Close 后仍白等 #4698: the [code-review] matrix adapter E2EE 解密失败静默丢弃消息：无 key re-request、无重试、无提示（cryptohelper 未使用，库不自动补 key） #4644 key-retry wait was a raw time.Sleep inside the serial syncer OnEvent callback — up to 2.4s per bad event during backfill batches, stalling every room behind it, and unbounded by ctx cancellation (doomed sleeps still waited out). Now a select over ctx.Done/time.After; [code-review] matrix adapter E2EE 解密失败静默丢弃消息：无 key re-request、无重试、无提示（cryptohelper 未使用，库不自动补 key） #4644 retry semantics preserved.

Tests

• TestIssue4697_ReadErrorAbortsSave (EISDIR path swap → Save must error, not full-write), ..._ExternalSectionFailureSurfaces (poisoned vendors.yaml → error mentioning external sections; baseline save still green).
• TestIssue4698_KeyRetryCtxAware (source pin: select ctx.Done/time.After inside the bounded loop).

Verification

• GOFLAGS="-p=1" GOMEMLIMIT=2GiB GOGC=50 go build -tags goolm ./internal/config/ ./internal/im/ green
• Full packages green: internal/config (13.7s), internal/im (50.3s)

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CI blocked by pre-existing main failure (not this PR's diff): the Test job's only failure is internal/tui TestHandleTunnelInboundMsg_WithImages panic (nil refusalLedger via ReleaseMatchingRefusals, refusal_ledger.go:761) — main's own latest run fails the same test (bca7ae7 #4726 suspect). This PR's config/im tests are green in the same run. Filed #4736 for the main-side crash. Per the CI hard rule this PR stays unmerged until main is fixed and the branch re-runs green; the fix commits themselves are verified locally (config+im full packages green, 3x targeted).