Use PT_LOAD p_paddr directly when translating /proc/kcore segments

[demoray]

kcore() previously computed each segment's physical address as
phdr.p_vaddr - (first_phdr.p_vaddr - first_iomem.start). That
assumes a single global virtual-to-physical offset across the entire
kernel address space — true on x86_64 (one direct map) but not on
PPC64 or any architecture where the kernel maps physical memory
through multiple non-contiguous virtual slabs. On those systems AVML
read from the wrong file offsets, producing corrupt acquisitions.

Use phdr.p_paddr directly. The kernel already populates it with the
physical address backing each PT_LOAD; no arithmetic translation is
needed. Skip the all-ones sentinel (u64::MAX on ELFCLASS64,
u32::MAX widened on ELFCLASS32) the kernel uses to mark kernel
virtual-only mappings (vmalloc, modules) that have no physical
backing. Skip zero-length segments defensively.

Pulled the translation out of kcore() into a free-standing
physical_ranges_from_segments helper so it can be unit-tested
without /proc/kcore.

find_kcore_blocks is unchanged: the iomem ∩ PT_LOAD intersection
that bounds acquisition to actual System RAM is preserved.

This addresses the same bug as #114 without that PR's regressions
(dropping the iomem cross-check, deleting the unit test, and the
silent zero-byte acquisition failure mode of the found_first gate).

Added five unit tests:

• physical_ranges_use_paddr_not_vaddr — segments with garbage
  p_vaddr relative to p_paddr still produce correct output. This
  is the regression PPC64 hits; the old code would have mistranslated.
• physical_ranges_skip_non_pt_load_segments — PT_NOTE etc. are
  filtered out.
• physical_ranges_skip_sentinel_paddrs — both the 64-bit and
  zero-extended 32-bit sentinels are filtered.
• physical_ranges_skip_zero_size_segments — defensive.
• physical_ranges_sorted_by_paddr — output sort order is what
  find_kcore_blocks requires.

Verified with:

• cargo clippy --all-features --all-targets -- -D warnings
• cargo clippy --no-default-features --all-targets -- -D warnings
• cargo test --all-features
• cargo fmt --check