Add JET.jl static analysis tests

[ChrisRackauckas-Claude]

Summary

• Add JET.jl as a test dependency
• Add test/jet.jl with static analysis tests for core functions
• Test getindex, setindex!, size, and setdiagonal! for runtime errors
• Test type stability of getindex and size

Analysis Results

JET analysis was run on the package and found:

• No runtime dispatch errors in the package code
• Type stability: Core operations (getindex, setindex!, size, setdiagonal!) are type stable
• The mul! functions show issues related to Julia's @threads macro internal assertions (not package code)

The existing codebase is already well-typed:

• Type-parameterized SparseBandedMatrix{T} struct
• Explicit type conversions with convert(T, val)::T patterns
• Duck typing for val and diagvals parameters is intentional for flexibility

Test plan

• Ran JET.report_package - no errors
• Ran JET.report_call on all key functions - no errors
• Ran JET.report_opt for type stability - core functions are type stable
• Ran Pkg.test() - all 27 tests pass

cc @ChrisRackauckas

🤖 Generated with Claude Code

[ChrisRackauckas-Claude]

CI Status Update

All test jobs passed:

• Tests (1) ✅
• Tests (lts) ✅
• Tests (pre) ✅
• test ✅

The failing jobs are pre-existing issues unrelated to this PR:

• Spell Check: Flags "BA" in comment # C = Cb + aBA on line 117 (pre-existing, represents mathematical formula aBA)
• build (format check): CI infrastructure issue with JuliaFormatter/SciMLStyle installation
• cleanup: Cleanup job failure

The JET.jl tests I added are working correctly on all Julia versions (1, LTS, pre-release).

cc @ChrisRackauckas