PHLEGM v6 Carrier-Relative Final Install Fix

Problem
- The manifest value upstreamInstallRelative=../../PHLEGM is defined relative to the Developer carrier/build root: the directory that contains the inner PHLEGM/ working directory.
- The bootstrap, Compiler UI fallback, and direct build fallback incorrectly resolved it from the inner PHLEGM/ project root. That shifted the live-install destination one directory too deep.

Correct contract
- Source build/project payload: <carrier>/PHLEGM/ (or verified archive ./**/PHLEGM/).
- Relative install target base: <carrier>/.
- ../../PHLEGM therefore resolves two levels upward from the carrier and then into the live PHLEGM/ directory.
- The live destination is merge-copied nondestructively: collisions are replaced, new files are added, and destination-only content is preserved.

Implementation
- External bootstrap resolves upstreamInstallRelative from $destinationCarrier.
- Shared Compiler UI fallback resolves it from $carrierRoot = parent(ProjectRoot).
- Build-PHLEGM-v6.ps1 direct/manual fallback resolves it from the same carrier root.
- The exact absolute path is still forwarded bootstrap -> Compiler UI -> build matrix.
- Recursive ./**/PHLEGM/ archive payload discovery and post-copy SHA-256 verification remain intact.
