Fix real-server CI environment and registry reuse

[segoon]

Summary

• keep server discovery on an isolated PATH while giving Mason package installers access to the workflow-provisioned toolchain through a dedicated installer path
• persist the planner Mason registry and share one immutable snapshot across all matrix shards
• validate and copy the snapshot into each isolated case home
• document the CI environment and registry snapshot behavior

Tradeoffs

Separating server lookup from installer lookup preserves the guarantee that --download exercises Mason even when the runner contains a same-named server proxy. It introduces one internal environment channel between the E2E harness and Mason installers; new installer subprocesses must use the shared installer-command helper. Sharing an artifact adds a small upload/download cost while eliminating per-shard registry discovery and reducing rate-limit exposure.

Validation

• 384 Rust tests passed; 4 ignored
• Clippy passed for all targets and features
• fake LSP fixture Clippy passed
• formatting, README generation check, workflow YAML parsing, and whitespace checks passed
• focused planner snapshot regression test passed
• rust/rust_analyzer lifecycle and smoke E2E cases passed using the isolated Mason server

The complete make test target reaches cargo deny and then fails on the existing rustls 0.23.40 advisory RUSTSEC-2026-0285. The advisory requires rustls 0.23.45 or newer and is unrelated to this patch.