[Feat] LSP server (xazz-lsp) reusing the checker

[ax1s-x1zz]

Context

docs/ROADMAP.md Track B-3. Xazz's headline value is "catch errors before execution", but today that only works via CLI or the IDE. Editor integration is the biggest DX/visibility win.

Scope

• New crate (tower-lsp) exposing diagnostics, hover, go-to-def, rename by reusing the checker
• Column-aware hover/autocomplete driven by the Typed IR's flowing Schema

Acceptance

Diagnostics in VS Code match xazz check line:col output exactly.

Dependencies

None directly (checker already returns structured spans). Enables E-1.

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See docs/ROADMAP.md for the full scale plan and execution order.