attribution: scope Mixed to the touching session and unify authorship rule

Two related correctness bugs in line attribution:

1. entire blame and entire why disagreed on the tag. resolveLine marks a line [MX] only when its preferred candidate is mixed, but the why-time enrichment path (enrichAttributionLineWithFetch) marked it [MX] if any candidate was mixed. A line with a non-mixed preferred candidate and a mixed secondary one rendered [AI] in blame and [MX] in why. Extract a single authorshipForPreferred() rule used by both paths.

2. Mixed was checkpoint-scoped, not session-scoped. readCheckpointContext OR-combined the checkpoint-wide CombinedAttribution with every session's attribution, so a line from an agent-only file got tagged [MX] whenever the checkpoint also touched a hand-edited file. Scope Mixed to the session whose work actually touched this file; fall back to the checkpoint-wide attribution only when no session metadata resolves.

Adds a regression test where the file-touching session is pure-AI but the checkpoint's combined attribution is mixed — the line must stay [AI].