Restore loadable scanner modules

[simsong-codex]

Codex-authored replacement for the abandoned removal approach in #523.

Restores runtime scanner loading through a versioned bulk_extractor_scanner_v1 factory, discovers scan_* modules from -P and BE_PATH, retains module handles until scanner cleanup, and documents the supported ABI. The new integration test builds a real module and verifies that -P discovery registers and enables it.

Validation:

• bash bootstrap.sh && ./configure
• make -C src check TESTS=test_be -j4 (pass; 68 test cases, 100456 assertions)

The broader make check also reaches the pre-existing sandbox-only machine_stats failure because this environment denies /bin/ps; it is unrelated to this change.

[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.29%. Comparing base (6654dd3) to head (6155b01).

Files with missing lines	Patch %	Lines
src/be20_api/scanner_set.cpp	83.33%	5 Missing ⚠️
src/test_be3.cpp	94.74%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #528      +/-   ##
==========================================
+ Coverage   74.18%   74.29%   +0.11%     
==========================================
  Files         115      116       +1     
  Lines       12928    12982      +54     
==========================================
+ Hits         9590     9644      +54     
  Misses       3338     3338              

Flag	Coverage Δ
unittests	74.29% <90.00%> (+0.11%)	⬆️

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[simsong]

scanner_set.cpp still has a bunch of #if 0 constructs in it that seem to be related to this functionality. Plese review and delete as necessary. Clean up the entire file as part of this PR.

Address codecov issues.