feat(platform): the Elevation trait and its three OS launchers (T40a)

[haiquang9994]

Summary

T40a: the Elevation abstraction in mixengine-platform and its three per-OS launchers.

• The Elevation trait plus a mock for tests.
• Windows: the UAC launcher.
• Linux: the pkexec launcher, with agent detection and a manual fallback.
• macOS: the osascript launcher, with the script kept as a constant.
• The three launchers' tables compile and are tested on every OS.

Tests

• Unit tests for each launcher table on all three OSes.
• The three system legs for the launchers, with the report reader turned into an assertion on the leg that must not have one.

Docs

• Design spec under docs/superpowers/specs/.
• T40a ticked in the roadmap, with what the macOS leg measured recorded.