Avoid invalidated tag slices in codec validation

[rtfeldman]

Derived parser validation held a borrowed tag-range slice across recursive type-store growth, so reallocation could corrupt the next tag's argument range and crash. Traverse tag and argument spans through stable indexed store access in both derived parse and encode validation, which also removes temporary per-tag allocations. Fixes #10695.

[greptile-apps]

Greptile Summary

The PR replaces borrowed tag slices and temporary argument copies with stable indexed type-store traversal in derived parse and encode validation.

• Iterates tag unions through getTagAt and getVarAt so recursive store growth cannot invalidate borrowed slices.
• Adds and registers a regression test for the derived parse crash involving repeated aliased tag-union fields.

Confidence Score: 4/5

The PR appears safe to merge, with the non-blocking caveat that the encode-side traversal change should receive equivalent regression coverage.

The indexed access APIs preserve stable ranges and values across type-store reallocations, addressing the invalid borrowed-slice path; the remaining concern is limited to incomplete test coverage of the parallel encode implementation.

Files Needing Attention: src/check/Check.zig, src/check/test/issue_10695_test.zig

Important Files Changed

Filename	Overview
src/check/Check.zig	Safely converts parse and encode tag-union validation to index-based traversal; the encode-side change lacks matching regression coverage.
src/check/test/issue_10695_test.zig	Adds a focused parse regression that checks the reported repeated-alias scenario completes without compiler errors.
src/check/mod.zig	Registers the new issue regression test with the check test suite.

_{Reviews (1): Last reviewed commit: "Avoid invalidated tag slices in codec va..." | Re-trigger Greptile}

[greptile-apps]

Encode traversal lacks regression coverage

The encode tag-union traversal now uses the same indexed access strategy, but the regression source invokes only Json.parse. Add an encode case with recursive or repeatedly aliased tag-union fields so an encode-specific traversal regression does not pass undetected.

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