fix(elf): preserve relative relocations by appending notes in-place

[littledivy]

Problem

Elf::append rebuilt the entire binary through object::build::elf::Builder, which reconstructs the file purely from its section table. That is lossy for modern relocatable executables:

• object 0.36 has no SHT_RELR support — Builder::read errors (Unsupported section type 13) on any binary carrying a .relr.dyn section header.
• Any segment bytes not covered by a surviving section are silently dropped on write.

So when a binary's section headers are stripped (as deno compile's release-linux base is), the .relr.dyn relative relocations survive only in a PT_LOAD referenced by DT_RELR — and the rebuild drops them.

The v8 149.4.0 bump added the first load-bearing relative relocation (a C++ static-init guard's mutex pointer). Left un-relocated, it deadlocks/aborts at startup:

libc++abi: __cxa_guard_acquire failed to acquire mutex

Only reproduces on release-linux-x86_64, where the strip is aggressive enough to leave .relr.dyn outside the section table. Verified locally against object 0.36.3: flipping a fixture section to SHT_RELR makes Builder::read fail, and Builder::write only round-trips section-backed bytes.

Fix

Replace the full rebuild with an in-place, patchelf --add-note style append that keeps every original byte:

1. Append the note payload + an enlarged copy of the program header table at a page-aligned offset past EOF.
2. Add a PT_LOAD mapping that region and a PT_NOTE pointing at the note, and repoint e_phoff/e_phnum + PT_PHDR at the new table (so the loader's load-bias math, AT_PHDR - PT_PHDR.p_vaddr, stays correct). This is what the runtime find_section/dl_iterate_phdr reads.
3. When the input still has a section header table, also add a real SHF_ALLOC SHT_NOTE .note.sui section — relocating + growing the section header table and .shstrtab past EOF — so the note survives a later strip (BFD tools rebuild from sections). A fully stripped binary keeps the note via PT_NOTE alone.

The original program/section header tables, .shstrtab, segment contents, and all relocations are copied (enlarged), never edited in place — so .relr.dyn survives unchanged.

Tests

• test_elf executes the produced binary on Linux CI — exercises the relocated PT_PHDR. ✅
• test_elf_note_survives_strip confirms the note is recoverable after GNU strip. ✅
• test_elf_note_mapped_and_preserves / test_elf_note_does_not_overlap_bss rewritten to verify the note via its PT_NOTE program header. ✅
• New test_elf_append_preserves_original_bytes: all bytes past the ELF header preserved, section table relocated + grew by one, program header table grew by two, SUI note discoverable. ✅

All four CI runners green.

Note: no version bump included — left for a follow-up release when deno bumps the libsui pin.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 94.59459% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.00%. Comparing base (cc8922e) to head (8e66f32).

Files with missing lines	Patch %	Lines
lib.rs	94.59%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #72      +/-   ##
==========================================
+ Coverage   68.95%   72.00%   +3.05%     
==========================================
  Files           3        3              
  Lines         963     1036      +73     
==========================================
+ Hits          664      746      +82     
+ Misses        299      290       -9     

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