Spike: unify name salting behind a single SaltMap

[asinghvi17]

Spike / for discussion — stacked on as/linux-no-patchelf (PR JuliaLang#150). Targets that branch, not main, so the diff shows only the salting change.

This addresses the review comment on JuliaLang#150 that the salting conventions are messy — the mix of plat_salted_basename (full-basename transform), replace_dep_libs's prepend/prefix convention (implicitly required to match), and the hooks taking the raw salt alongside already-salted inputs.

Approach

Build one explicit name→salted-name description once per run and thread it everywhere, so the salted form of a name is derived in exactly one place:

struct SaltMap{F}
    salt::String                      # retained only as a per-run record (logging)
    rename::F                         # the one definition of "how a name is salted"
    basenames::Dict{String,String}    # original ⇒ salted, for files found on disk
end

• rename is the platform's transform; it's total (works on basenames, bare SONAME/NEEDED strings, and the extension-less dep_libs stems).
• basenames records which names were salted, for dependency/NEEDED rewrites and symlink retargeting.
• They're consistent by construction (basenames[k] == rename(k)).

What this removes

• plat_salted_basename → folded into plat_make_renamer (the sole salting definition).
• replace_dep_libs's prepend::Bool + prefix coupling → deleted; salt_dep_libs tokenizes the blob and applies rename per stem (verified byte-identical to the old whole-blob replace on both platforms).
• Raw salt + already-salted inputs in the rewrite hooks → plat_set_library_id! takes the SaltMap and resolves its own id; plat_install_name_change! takes fully-resolved old/new. plat_dep_prefix collapses into a single plat_dep_ref(platform, name).

Also deleted: plat_dep_libs_prepend. macOS vs Linux now differ only in how the map is built — macOS prepends (<salt>_libjulia, may grow, @rpath/<name>), Linux substitutes a same-length token (libjulia → <salt>, length-preserving, bare <name>).

Validation

• test/elf_ops.jl: 47/47, including the synthetic testset (30 assertions) that drives privatize_libjulia_common! end-to-end against the real libjulia.so/libjulia-internal.so and asserts no library keeps an unsalted SONAME/DT_NEEDED.
• macOS is preserved by construction but not run (no macOS host) — the one spot to eyeball is the @rpath/basename(salted_path) id-set.

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