[mhutchinson]

Adds the foundational leaf packages for vindex:

• vindex/internal/wire:
  • Encoders and strict parsers for c2sp.org/tlog-vindex-state@v1 (Output Log state commitment entries), c2sp.org/tlog-vindex-keys@v1, c2sp.org/tlog-proof@v1 binding stanzas, and c2sp.org/tlog-vindex-values@v1 backward-paginated value log responses per vindex/docs/tlog-vindex.md.
  • Canonical key validation (ValidateCanonicalKey), RFC 3986 percent-encoding (PercentEncodeCanonicalKey), and origin validation (ValidateInputOrigin).
  • RFC 6962 value log hashing and compact range root reconstruction (LeafHash, InteriorHash, ComputeValueLogRoot) via github.com/transparency-dev/merkle.
  • Checkpoint and witness policy verification (Policy, BuildPolicy, ParsePolicy, ParsePolicyOrVerifier, ParseVerifier, ParseWitnessVerifier, and MTCVerifier) backed by github.com/transparency-dev/formats/{log,note,policy,proof}.
• vindex/internal/metrics:
  • Prometheus metrics definitions for progression gauges, stage throughput counters, pipeline latency histograms, and independent auditor health gauges per vindex/docs/ARCHITECTURE.md.
• go.mod / go.sum:
  • Updates github.com/transparency-dev/formats and tidies go.mod to only the dependencies used in the repository at this commit.