Support multiple mastering items per track

[lsooxlla8]

Summary

• update Prepare Selected Items for Mastering to 1.3.0
• treat all selected items on one track as one mastering block
• preserve relative item positions and gaps when arranging blocks
• span each region from the earliest selected item start to the latest selected item end
• derive render channels from every selected source in the track block

Validation

• parsed every Tracks and Toolbars Lua script with luaparse
• checked whitespace with git diff --check
• verified a three-item track block preserves offsets and uses the full region span
• synchronized the locally installed REAPER copy and verified matching SHA-256