Resolve duplicate Codewith rollouts deterministically

[andrei-hasna]

Problem

The same Codewith source ID may appear in live, copied, archived, or continued rollout files. Unsorted enumeration plus last-writer behavior can let stale copies win.

Acceptance

• Define and document canonical-file precedence or deterministic continuation merge.
• Filesystem enumeration order cannot change the result.
• Live and archive roots do not create duplicate sessions.
• Newer complete data cannot be replaced by stale partial data.
• Duplicate fixtures produce stable content and metadata.

Todo: OPE2-00079