docs: clarify local Rust rebuild cache behavior

[shinaoka]

Summary

• document the local ../sparse-ir-rs rebuild flow in the Julia README and deps README
• note that Julia precompile cache can keep pointing at the artifact-provided library after Pkg.build()
• document removing ~/.julia/compiled/.../SparseIR and restarting Julia when that happens

Test Plan

• julia -e 'using Pkg; Pkg.build()'\n- julia --project=. -e 'using SparseIR; println(SparseIR.C_API.libsparseir)'

[terasakisatoshi]

ERROR: LoadError: failed process: Process(/opt/hostedtoolcache/julia/1.12.5/x64/bin/julia -C native -J/opt/hostedtoolcache/julia/1.12.5/x64/lib/julia/sys.so -O0 -g1 --color=no --startup-file=no --project generate_C_API.jl, ProcessExited(1)) [1]

This error is resolved by #123