Recover a contiguous replayed prefix in packed session-log rows

[cloga]

We observed a persisted Zstandard JSONL session that could not resume. The surfaced error was complete frame contains a torn JSONL record, but byte-level inspection showed all 20,581 Zstd frames were complete and the plaintext ended on a newline.

The actual first invalid decoded range was:

• committed events ended at seq: 106277
• the next packed tool-call-chunks row decoded to seq: 106276..106320
• only 106276..106277 overlapped; 106278..106320 was the valid continuation
• there were no later gaps or overlaps, and the complete artifact decoded through seq: 522302 / turn/end

A manual recovery backed up the original artifact, decoded packed storage rows, discarded only the two-event duplicated prefix, rebuilt the Zstandard log, and verified 522,303 contiguous events (0..522302).

Proposed conservative behavior: when a decoded storage row starts before the current sequence cursor but reaches it contiguously, discard only the already committed prefix and consume the remaining suffix. Continue rejecting gaps and non-contiguous overlaps. This makes replayed packed appends idempotent without masking arbitrary corruption.

A tested fork implementation is available at cloga#13 and is tracked at cloga/dsh-windows-ops#38. Focused JSONL/Zstd tests pass (236 tests) and the host build/typecheck passes.

[xiaoshenming]

这个形状（packed 行从已提交 seq 往回重叠一小截、后面连续到文件末尾）我们默认 不会自动吃掉重叠前缀再接上后缀——那是发明「哪一段是真的」。当前 repair 在认不出崩溃恢复闭包时，仍裁到第一个 committed 缺口前的完整 turn/end。

dsh-session-surgeon 的 inspect 现在会保留 overflow（缺口后的行），dry-run 能看到会丢掉多少。你这种「只丢 2 条重复、保住 52 万后续」的手工选边，请先备份、用你 fork 里已经验证过的路径；不要对同一文件连 apply 两次。

dsh plugin --profile web add "github:xiaoshenming/dsh-session-surgeon#main"

[xiaoshenming]

补一刀：这个形状现在默认会修。

packed 行从已提交 seq 往回重叠、但连续接到当前游标时（你写的 106276..106277 已提交、106278..106320 是真续写），丢掉已经提交的前缀成员、收下尚未提交的后缀。序号本来就在磁盘上，不是发明 seq。后面如果还有真正的空洞，仍然裁到缺口前最后一个完整回合。

已推到 main。请再装一次后对备份 dry-run，看 actions 里有没有 packed-overlap-suffix：

dsh plugin --profile web add "github:xiaoshenming/dsh-session-surgeon#main"

不要对已经手工改过、又被第二个写者追加的文件连 apply 两次。健康会话点 repair 仍然不改字节。

[cloga]

确认了：cloga/deepseek-harness#13 已覆盖这里描述的 packed 重叠形状。

除了 PR 里的 scanLog 单测，我又补跑了一个 Zstandard 端到端回归：先落盘到 seq 2，再追加一条完整的 tool-call-chunks packed 行 seq 1..4。读取器会逐项确认 1..2 与已提交事件完全一致，丢弃这段重复前缀并保留 3..4；随后会话可以继续 append，既有 Zstd 帧不会被重写。JSONL 与 Zstd 两组持久化测试共 239 个通过。

因此 Discussion 中 106276..106277 重叠、106278..106320 为连续后缀的情况，属于同一条已覆盖路径。这个结论验证的是 Harness runtime reader；外部 surgeon 的 packed-overlap-suffix action 是另一条修复工具路径。

[xiaoshenming]

跟上了：packed 重叠前缀现在必须和已提交事件一致才会收下后缀。

和 runtime reader 同一条规则——重叠成员逐项对得上才丢前缀、留 3..4；对不上就当普通 seq gap，不选边。main 已更新（439fb81）。dry-run 仍看 packed-overlap-suffix。