diagnose weak HNSW topology failures at CASS scale

[lifeofgurpreet]

Summary

• Preserve the weak-connectivity admission gate and add bounded failure telemetry: weak-component count, zero-base-degree count, and the first zero-degree origin.
• Add a regression at CASS quality-tier cardinality (2,573,003) showing that the existing union-find accepts a fully forward-attached insertion chain at production scale.
• Extend malformed-topology assertions to pin the diagnostic fields.

This distinguishes a validator scale fault from a construction/exposure fault without relaxing the integrity gate.

Validation

• cargo fmt --check --package frankensearch-index
• cargo metadata --no-deps --format-version 1
• Targeted cargo test could not complete locally because this checkout eagerly compiles unrelated workspace/dev test dependencies (including the sibling fast_cmaes path dependency, asupersync, Criterion, and HDF5). The attempted builds were stopped before tests executed to avoid consuming host disk; CI remains the authoritative test run.

Related to #32; production evidence is in issue comment 5324416151.