fix(runtime): preserve compaction projection after overflow

[likun666661]

Summary

• allow reactive step-0 context overflow recovery to compact prior history as a pre-turn checkpoint while preserving the current user anchor verbatim
• pin checkpoints accepted during an active send and rebuild every later provider request from the raw durable ledger through that checkpoint boundary
• fail closed on checkpoint/source mismatches so compacted raw history cannot silently reappear
• add regression coverage for overflow -> compact retry -> tool call -> successor request

Why

The reactive compactor previously required a safe span after the current user anchor. At step 0 no such event exists, so recovery failed with no_safe_completed_span even when prior history was foldable. When recovery did succeed later, its compacted messages only applied to the immediate retry; a following tool step rebuilt from the cached prior replay and could resurrect the raw covered span.

Testing

• npm --workspace @maka/runtime run typecheck
• Biome lint on the five changed files
• 74 related tests across overflow recovery, mid-turn compaction, and history checkpoint replay