* fix(server): keep diff highlighting responsive and bounded
Tiny patches previously parsed both full source versions synchronously. Yield between parser work units and admit source by UTF-8 bytes, keeping large patches renderable without tokens. Cancel obsolete highlighting when a newer diff supersedes it.
* fix(server): keep diff cancellation with its requester
Independent commit viewers must not cancel each other. Use request-source cancellation for commit highlighting and cache invalidation for checkout highlighting. Oversized source versions use the existing budgeted hunk reconstruction so large lockfiles retain useful syntax tokens.