* fix(native): retain value trace source evidence
* fix(native): build trace error evidence only on failure
* test(mcp): expect unowned value trace refusal
* test(mcp): avoid freezing trace error wording
* test(native): keep value-trace stop coverage with source evidence
Main's stop and continuation cases now meet the trace's evidence-owner
precondition: direct calls supply a source id, and a completed MCP trace
runs in a session that can retain it.
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Co-authored-by: Cursor Agent <cursoragent@cursor.com>