Fix incorrect file offset calculation in memory mapping

[pablogsal]

The current implementation incorrectly assumes that calculating a file
offset from a process memory address can be done using a simple
subtraction from the library's load address. This assumption doesn't
hold for binaries with non-standard ELF layouts, where PT_LOAD segments
may have different virtual address to file offset mappings.

Fix the issue by:

1. First converting the absolute process address to a library-relative
   offset by subtracting the library's load point in the process
2. Finding the PT_LOAD segment in the ELF file that contains this offset
3. Using the segment's p_vaddr and p_offset to calculate the correct
   file offset

To avoid performance penalties from repeatedly parsing ELF files, add
caching of PT_LOAD segments per library.

Example of what was wrong:
old: file_offset = addr - lib_start
new: file_offset = ((addr - lib_start) - segment->p_vaddr) + segment->p_offset

This fixes an issue where pystack would read from incorrect file offsets
when analyzing binaries compiled with non-standard layout options (e.g.,
when using the gold linker with custom flags).

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 59.61538% with 21 lines in your changes missing coverage. Please review.

Project coverage is 83.33%. Comparing base (19b9759) to head (def56f4).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/pystack/_pystack/mem.cpp	73.80%	11 Missing ⚠️
src/pystack/_pystack/process.cpp	0.00%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #220      +/-   ##
==========================================
- Coverage   83.51%   83.33%   -0.19%     
==========================================
  Files          46       46              
  Lines        6201     6248      +47     
  Branches      134      458     +324     
==========================================
+ Hits         5179     5207      +28     
- Misses       1020     1041      +21     
+ Partials        2        0       -2     

Flag	Coverage Δ
cpp	83.33% <59.61%> (+18.89%)	⬆️
python_and_cython	83.33% <59.61%> (-15.73%)	⬇️

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[godlygeek]

The commit messages for the fighting-the-CI commits need rewording to explain the rationale for the changes. Other than that...

[godlygeek]

OK, I've pushed a bunch of fixup commits for minor improvements, plus one bug fix for a bug that I think has existed for as long as pystack. Please review my changes, and if you're happy with them feel free to squash and land this. Let me know if you dislike any of the fixups, or if you disagree with my diagnosis of the range check bug.

[godlygeek]

Oh, and the fixup commits actually have commit message bodies, so each one should explain the rationale for the change.