Problem

When history loaded a provider compaction whose native.messages didn't decode as @opencode/ai messages, the whole read failed with MessageDecodeError. That made the session unusable. Since #57241, a compaction hook's result.native is stored as-is, so one malformed plugin result could break a session for good.

Change

A window that doesn't decode is now treated like another model's window: it's skipped, and history falls back to the messages before it.

load history for model M
  latest compaction replayable on M?   (no native window, or same model AND window decodes)
    yes -> read from it, as before
    no  -> read from the compaction before it   (new: `latestCompaction(..., before)`)

• decodeMessageRow no longer validates the window, so model-neutral readers such as store.context and the UI don't fail on it.
• The window is checked only where it would be replayed (replayable). Replay already decoded it, so the cost is the same as the validation it replaces.

Verification

packages/core/test/session-native.test.ts "skips a malformed native window and replays the history before it" replaces the test that expected MessageDecodeError. With a malformed window stored between two prompts:

• history replays both prompts;
• store.context loads.