Prepare v5.1.0-rc18

[niksedk]

Adds the v5.1.0-rc18 change log section and bumps Se.Version.

The entry set was compiled by ancestor-checking every merged PR's merge commit against the v5.1.0-rc17 tag — 13 PRs merged since it: #12874, #12876, #12881, #12882, #12883, #12884, #12885, #12887, #12890, #12891, #12895, #12896, #12897.

Two are not listed separately:

• Set sync point: reuse the "Video position" string for the time code box #12897 is a string-reuse refactor of Add sync point time code box to Set sync point (SE4 parity) #12896, so both are covered by the single "Set sync point" line.
• Replace remaining object lock fields with System.Threading.Lock #12884 (remaining object lock fields → System.Threading.Lock) is internal only, with no user-visible effect.

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