Rework weight_consensus onto contract Backend protocol

[LandynDev]

Part of repo-registration phase 2a (task 2a-4). Stacked on #1666 (python client) — merge order #1665 -> #1666 -> this.

Re-lands the consensus core from the closed commitments-based attempt (#1651's successor branch) reworked onto an injected Backend protocol with the repos-v0 contract as transport:

• backend.py — ConsensusBackend protocol: fetch_registry / fetch_baskets / fetch_own_basket / publish_basket / fetch_stakes_and_permits (spec S4; metagraph reads behind the same protocol per S5)
• contract_backend.py — ContractBackend over RepoRegistryContractClient; snapshot block number resolves to a block hash once and every childstate read pins to it (S1 — closes the reorg gap the spec calls out), publish via set_basket signed by hotkey (S3)
• consensus.py — cherry-picked verbatim except the input seam: structured baskets + validate_prefs replace bytes + decode_prefs (R3: second decode site is now validate-only)
• codec.py — canonicalize_prefs kept verbatim (R4); zlib wire format, SCALE payload walker, CONSENSUS_MAX_PAYLOAD_BYTES, CONSENSUS_FRESH_WINDOW_BLOCKS deleted
• chain.py (commitments transport) deleted; mirror_sync.py NOT re-landed — mirror reconciliation moved to das-github-mirror fix: repo names #229; validator has zero mirror-write responsibilities, MirrorClient stays read-only
• neurons/validator.py — consensus manager init is failure-safe: missing contract address or init error degrades to baked weights, never blocks startup
• Manager cache/fallback ladder, apply_consensus, postgres storage unchanged per spec

Decisions flagged for review (details in code):

• basket github_id -> name resolution happens inside ContractBackend at the same pinned hash (deterministic across validators); deregistered ids dropped, per-voter renormalization absorbs the weight
• publish filters unregistered repo names with a warning and re-quantizes survivors; skip-if-equal prevents tx spam

Tests: +67 (golden aggregation vectors, fallback ladder via FakeBackend, pinned-hash propagation, validate-only decode, publisher edges); full suite 1104 passed; ruff clean.