Harden build prefix relocation of poured bottles

[MikeMcQuaid]

• Keg#relocate_build_prefix NUL-pads raw prefix strings in pinned bottles at pour time but had no tests beyond one happy path; the hardening cases in the plan each get a spec, and three found bugs.
• Patching wrote the file atomically, giving it a new inode, so any hardlinked names kept the unpatched content. Files are now grouped by inode, patched once and re-linked afterwards, as text relocation already does.
• A prefix longer than the bottled one failed midway with a size mismatch after earlier files had already been rewritten; it is now refused up front.
• Linkers suffix-merge strings, so a dynamic string table entry can be referenced from its interior (libfoo.so inside /old/prefix/libfoo.so); trailing NUL padding shifted such references onto garbage. Strings with interior loader references are now padded with extra path separators after the prefix, which path resolution ignores, keeping every reference's offset valid. Other strings keep NUL padding.
• Also covered: several occurrences per string and several strings per file, the sharball skip, re-signing of patched files and prefix strings inside Mach-O load commands.

This change is part of plans/relocatable-bottles.md

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• I did not use AI/LLM to create this PR, or I disclosed the tool/model below and reviewed its output; I did not attribute commits to AI and will answer maintainer questions and review comments myself without AI/LLM.

Claude Fable 5 high with local review and testing.

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