chore(distribution): review upstream compiled installs, migration, and rollback

[rynfar]

Prime upstream now defaults supported macOS/Linux installs to compiled binaries, migrates existing npm installations, and adds verified update/rollback. This overlaps Pylon's signed managed distribution and the public SDK package Pylon loads.

Evaluate the complete dependent stack together:

• #2140: compiled installers, dd760d3.
• #2165: npm installation migration, 5a3b922.
• #2166: verified updates and rollback, 011574e.

Acceptance:

• Make an explicit adopt/hybridize/retain/redesign decision for upstream installation/update behavior versus Pylon-managed distribution.
• Verify importable public SDK exports, detached-daemon launch, negotiated features, owned-session cleanup, and managed Full access restart adoption remain supported.
• Ensure automatic upstream migration or self-update cannot replace a Pylon-selected immutable runtime outside Pylon's maintenance flow; preserve stock installations and user state.
• Assess supported platforms and fallback behavior, migration from existing npm versions, failed/interrupted downloads, concurrent updates, and offline rollback.
• Preserve protected publication, artifact verification, and Pylon's install/update/switch-back ownership. If adoption changes managed artifacts, publish and validate a new immutable build through the existing release gates.
• Run focused packaging/installation regressions and hold one integration checkpoint after the combined change is ready.

Suggested priority: review compatibility before scheduling implementation.

Backlog follow-up under #1 and Pylon's Prime integration tracker. The completed September integration remains the baseline; this issue tracks later upstream adoption. Before implementation, refresh upstream evidence and record the ownership decision and focused verification in .pylon/features.yaml and .pylon/upstream-review.md.