fix: rewind vs compaction boundaries + failed requests as timeline runs (v0.21.0)

[lroolle]

Two session-view truth bugs reported from one real trace pair, both reproduced and fixed against the actual wire data.

1. A /rewind displayed as "compacted"

The post-spine anchor walk accepted a msg[0]-to-msg[0] match with zero verified context. msg[0]'s content sig is the injected system-reminder prefix — identical for every request in a session — so a rewind-to-start (71 -> 2 turns, only the first message shared, everything after divergent) classified as a fold compaction.

Fix, in two parts:

• The degenerate (i=0, j=0, checked=0) anchor is rejected. Anchors elsewhere may still verify zero neighbors (wildcard tool results at start-of-packing) — the real-wire compaction fixture has one and it stays accepted.
• The boundary classifier reads index geometry: a fold's surviving tail aligns at shifted indices (the history above it shrank into a summary); a rewind's shared content is a same-index prefix. With no anchor at all, a same-sig thread that was never reunified classifies rewind too — every observed compact shape rewrites msg[0] and splits the sig, so merged continuation requests are now stamped and only they can claim a full rewrite.

The boundary renders "rewound · N -> M turns" (rail row + convo divider) with a hover that says what actually happened. Fold and rewrite are unchanged; tests/session-compaction.test.ts (real wire shapes) pins all of it.

2. Failed requests vanish or dump at the tail

Transport/HTTP failures transiently claimed the successful retry's turn at strength 1, then piled up as orphan "err" rows at the thread tail — a real kimi 429 storm rendered 82 unordered rows. Now they collect per timeline position (t.failed), never claim turns, and render as one collapsed run at the exact position the storm hit: "21 failed requests · 429 engine_overloaded_error", red dot on the rail, first wire pair linked, matching red-edged line in the conversation pane.

Verification

• 486/486 tests pass (5 new session-layer tests, 2 new dom-stub UI tests)
• Both real traces re-rendered through the actual page: the rewind trace shows 1 "rewound" boundary and 0 "compacted"; the kimi trace's chat shows its 3 storm runs (2/1/5) collapsed in position, zero page errors
• Binary compiles clean

Releases as v0.21.0.

🤖 Generated with Claude Code