feat(inlay-hint): implement deterministic definition tracing for pkgs

[Malix-Labs]

FIXES: #835

This PR implements deterministic definition tracing for the pkgs scope to resolve issue #835.

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Agent Information

• Name: Gemini 3.5 Flash (High)
• Environment: Antigravity-CLI
• Review Environment: Zed

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What I Was Instructed to Do:

1. Fetch and read GitHub issue Package Version Inlay Hints Does Not Work Without with pkgs; #835.
2. Develop a technical action plan analyzing the bug and potential solutions.
3. Implement Option 2 (Deterministic Definition Tracing) to resolve package inlay hints.
4. Ensure the code is highly qualitative, minimal, DRY, and optimized.
5. Create a draft git commit for the changes and submit a draft PR.

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Summary of Work:

1. Deterministic Value Tracing: Added the resolvesToPkgs helper function to nixd/lib/Controller/AST.cpp. It traces expressions back to the Nixpkgs root by:
   • Resolving let-bindings (DS_Let) and recursive attribute bindings (DS_Rec).
   • Tracing import <nixpkgs> or import calls referencing Nixpkgs paths.
   • Checking select expressions (like inputs.nixpkgs.legacyPackages.${system}).
2. Heuristic Scope Update: Rewrote havePackageScope in nixd/lib/Controller/AST.cpp to use resolvesToPkgs, cleanly removing the old name-based heuristics.
3. Idioms Resolution Update: Updated mkVarSelector in nixd/lib/Controller/AST.cpp to return an empty selector (Nixpkgs root) for any traced variable resolving to pkgs.
4. AST Selection Traversal: Updated the dfs traversal in nixd/lib/Controller/InlayHints.cpp to handle Node::NK_ExprSelect, resolving select paths using the updated idioms selector and preventing duplicate lookups on the base.
5. Verification: Ran nix build and nix flake check to confirm everything compiles successfully without errors.