Handle known ARM64 libc6 usr-merge diversion warnings

[ibrahim halatci (ihalatci)]

Summary

Recognize the known protective libc6 ARM64 usr-merge diversion before Debian warning filtering discards its context. This is limited to complete, contiguous diagnostic blocks containing only these exact lines:

dpkg-shlibdeps: warning: diversions involved - output may be incorrect
 diversion by libc6 from: /lib/ld-linux-aarch64.so.1
 diversion by libc6 to: /lib/ld-linux-aarch64.so.1.usr-is-merged

An accepted block must have equal from/to counts, one warning per detail, and no prefix with more details than preceding warnings. This accommodates parallel dh_shlibdeps interleaving, including the actual topn/Noble warning / warning / to / from sequence captured in the new 26-line fixture.

Unknown owners, paths, architectures, local diversions, malformed or extra diagnostic records, missing context, imbalanced blocks, and detail overruns are not exempted. Unrelated output and blank lines end a block; context is never borrowed across boundaries. Only matching base-warning indexes are skipped, preserving raw build diagnostics and existing lintian state transitions. RPM handling, ignore-file semantics, --output_validation, system diversions, and the validate_output interface used by both callers are unchanged.

Local/offline evidence

• The existing warning suite passes 67 cases, including all 10 original cases unchanged and focused positive/negative coverage.
• The two changed Python files pass Black and the existing Prospector profile with --no-autodetect (zero findings).
• All six original build.stdout.log inputs for Citus, hll, and topn on Noble and Trixie were replayed through the unchanged standalone packaging_automation.validate_build_output CLI with their exact original inputs/packaging_ignore.yml files. Each returned 1 before the fix and 0 afterward, with no remaining unignored base or package-specific warnings. SHA256 comparisons confirmed all source logs and policies remained unchanged.
• Validation ran locally in WSL; no native package rebuild or runtime-installability result is claimed.

The original logs come from packaging diagnostic run 34258995794, attempt 1. That diagnostic run was red: its original warning gates failed, and collection also encountered a separate source-identity assertion. It is evidence for the original diagnostics and offline comparison, not a new green native run for this change.

Rollout scope

This draft changes only the warning handler, focused tests, and the small real-output fixture. A tools release/tag and packaging-pin updates require separate approval and rollout; this PR does not claim that currently pinned packaging builds are already unblocked.