docs(design): internal xxHash64 file-dedup heuristic

[oferchen]

Summary

• Adds docs/design/xxhash64-file-dedup.md: internal-only heuristic that pre-screens identical files before signature/delta computation.
• 64-bit xxHash64 fingerprint over (size, mtime, first 4 KB, last 4 KB) computed once at flist build time, then probed against same-flist siblings.
• Strict three-way gate (fp + size + mtime) with conservative false-negative handling: any mtime divergence falls through to the existing pipeline. Wire output is byte-identical with dedup on or off.

Test plan

• Doc renders correctly on GitHub (tables, fenced blocks).
• Wire-compat invariants (Section 8) line up with feedback_no_wire_protocol_features.md and the parent zsync design notes.
• Implementation / property-test / bench scaffolding PRs (follow-ups under Enforce single branding binary in workspace metadata #2102) reference back to this note.