Add branch/PR integration audit script, runbook, and Make target

[mdheller]

Motivation

• Provide a lightweight, deterministic audit step to verify ancestry and PR lineage before merging multi-day branch/PR work into main to avoid dropped commits or merge drift.
• Capture an operational, low-risk runbook so integrators follow a repeatable checklist that preserves evidence and enables quick rollback anchors.
• Surface merge-state guidance (SAFE FAST-FORWARD, ALREADY INTEGRATED, DIVERGED) so merges are chosen with clear intent and minimal surprises.

Description

• Add tools/audit_branch_pr_integration.sh, a POSIX-compatible script that compares a source ref and a target branch, lists merge commits, counts missing commits, computes the merge base, and classifies the merge status.
• Add docs/vnext/integration/branch_pr_merge_runbook.md with preconditions, the audit-first workflow, recommended merge commands, post-merge checks, and recommended recovery tagging.
• Update docs/vnext/integration/README.md to link the new runbook for discoverability.
• Add a Makefile target integration-audit that runs ./tools/audit_branch_pr_integration.sh main HEAD to standardize invocation from the repo root and set executable permissions on the script.

Testing

• Ran ./tools/audit_branch_pr_integration.sh work HEAD, which executed successfully and reported a safe fast-forward state for the inspected refs.
• Ran make integration-audit, which failed early and reported that the local main branch did not exist (this is an expected environment-state failure to be resolved by fetching/creating main).
• Attempted make verify, which failed to complete due to external dependency/network limitations: Rust cargo failed to download crates (HTTP 403), Go tests failed due to missing go.sum entries for golang.org/x/crypto/chacha20poly1305, and the Python fixture verifier failed due to a missing cryptography package.
• The above failures are environment/dependency related and do not reflect the changes to the audit tooling and runbook, which exercised correctly in the repository context.

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