BE-315: HashQL: Add GraphReadFilter source variant to MIR

[indietyp]

🌟 What is the purpose of this PR?

Add support for graph read filter closures in MIR by introducing a new GraphReadFilter source variant. This allows the compiler to properly handle and optimize filter closures used in graph read operations.

🔍 What does this change?

• Add a new Source::GraphReadFilter variant to represent MIR generated from filter closures in graph read operations
• Update the transform_closure method to accept a source parameter instead of constructing it internally
• Modify the inlining analysis to never inline graph read filters
• Replace the filters method in the callgraph with direct source type checking
• Update pretty printing to properly display graph read filters
• Ensure graph read filters are properly handled throughout the MIR pipeline

Pre-Merge Checklist 🚀

🚢 Has this modified a publishable library?

This PR:

• does not modify any publishable blocks or libraries, or modifications do not need publishing

📜 Does this require a change to the docs?

The changes in this PR:

• are internal and do not require a docs change

🕸️ Does this require a change to the Turbo Graph?

The changes in this PR:

• do not affect the execution graph

🛡 What tests cover this?

• Existing MIR tests should cover this functionality

❓ How to test this?

1. Checkout the branch
2. Run tests that involve graph read operations with filters
3. Confirm that filters are properly handled and not inlined

[cursor]

PR Summary

Medium Risk
Touches MIR reification and inlining classification logic; mis-tagging filter closures could change optimization behavior or call graph assumptions, but scope is limited and covered by updated golden MIR tests.

Overview
Adds a new MIR body origin, Source::GraphReadFilter(HirId), to distinguish graph-read filter closures from normal closures.

Updates reification so transform_closure/lower_closure accept an explicit Source, and graph-read filter lowering now produces bodies tagged as GraphReadFilter (treated as fat pointers like closures/ctors).

Adjusts inlining and analysis to reflect the new source: GraphReadFilter bodies are marked Never inline in cost analysis, the inliner’s filter set is built by checking body.source (removing CallGraph::filters()), and MIR text output renders filters as graph::read::filter@.... Golden UI test outputs are updated accordingly.

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Project	Deployment	Actions	Updated (UTC)
hash	Ready	Preview, Comment	Feb 2, 2026 4:42pm
petrinaut	Error		Feb 2, 2026 4:42pm

2 Skipped Deployments

Project	Deployment	Actions	Updated (UTC)
hashdotdesign	Ignored	Preview	Feb 2, 2026 4:42pm
hashdotdesign-tokens	Ignored	Preview	Feb 2, 2026 4:42pm

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[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.93%. Comparing base (784af93) to head (58566d5).

Additional details and impacted files

@@                                          Coverage Diff                                           @@
##           bm/be-301-hashql-size-estimation-for-local-variables-and-functions    #8323      +/-   ##
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+ Coverage                                                               83.92%   86.93%   +3.00%     
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  Files                                                                     293      154     -139     
  Lines                                                                   43595    21334   -22261     
  Branches                                                                 1178      539     -639     
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- Hits                                                                    36588    18546   -18042     
+ Misses                                                                   6588     2584    -4004     
+ Partials                                                                  419      204     -215     

Flag	Coverage Δ
rust.hashql-compiletest	46.65% <ø> (ø)
rust.hashql-core	?
rust.hashql-eval	?
rust.hashql-mir	89.22% <100.00%> (-0.01%)	⬇️

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