intel_mac: make trailer format name-aware and support chaining

[nathanwhit]

The Intel-mac path appends a sentinel-terminated trailer to the end of the binary, with find_section searching backward from EOF for the sentinel and returning whatever data follows it. Before this change the trailer stored only [sentinel | data_len | data] — no section name — so find_section ignored its section_name argument and returned the most recently appended data regardless of which name the caller asked for.

That breaks any binary that has more than one appended section. The concrete failure case: a deno release binary has a dnclbk trailer appended by the deno release pipeline; the user then runs deno compile, which appends a d3n0l4nd trailer. Inside the produced binary, find_section("dnclbk") would return the d3n0l4nd (last appended) data, and the residual-lazy-source loader would then index a 1.3MB-shaped manifest into a much smaller blob and panic.

Fix:

• New trailer layout (per section, chained at end of file):

  sentinel  16 bytes  '<~sui-data~>' (12) + magic (4)
  name_len   1 byte   u8
  name    name_len bytes (UTF-8, no NUL)
  data_len   8 bytes  u64 little-endian
  data    data_len bytes
  

• Two magic values are recognized: MAGIC (0xDEADC0DE) for the new name-bearing format, LEGACY_MAGIC (0xEFBEADDE) for the older name-less format (treated as matching only the empty name).

• find_section(name) walks backward across multiple trailers: if a trailer's name doesn't match, scanning continues strictly before its sentinel.

• Macho::write_section and Macho::build on x86_64 now carry the section name through and emit it in the trailer.

• Added unit tests for chained sections and legacy-format fallback.

Existing tests pass unchanged.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 77.14286% with 32 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.95%. Comparing base (92f691a) to head (562412b).

Files with missing lines	Patch %	Lines
intel_mac.rs	75.57%	32 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #61      +/-   ##
==========================================
+ Coverage   65.16%   69.95%   +4.79%     
==========================================
  Files           3        3              
  Lines         864      942      +78     
==========================================
+ Hits          563      659      +96     
+ Misses        301      283      -18     

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[littledivy]

Have you tested this on an intel mac? This code is fragile, ref #43