v0.6.7 — fixes on the chain-18 build

v0.6.7 — fixes on the chain-18 build (rolls onto chain 18 node by node)

v0.6.7-rc1 (the gas model, constraint set 8 — chain 18's cut build) plus fixes that change no
consensus rule, wire format, verifier key or shipped program digest. Runs chain 18 (genesis
a7cb020c…); refuses chains 14–17.

• #51 Sealed-proof pruning keeps a height index of seal marks and shrinks the block row with the
  pruned record (dormant: no chain aggregates).
• #64 Wallets decide the note-envelope format by chain id on chains 14–17 (LEGACY_ENVELOPE_CHAIN_IDS),
  never from a node's envelope_bytes claim there. Chain 18 is memo-on (1 860-B envelopes).
• #65 The node's viewing-key registry is keyed by a one-way id, not by the viewing key.
• Circuits aeacf31 = cs8 18c2627 + #49 (GPU kernels without aliasing &mut, H100-tested),
  #63 (rVM DSL allocator fixes; all shipped rVM programs byte-identical), #66 (ALU coverage tests).

Built on Linux (Ubuntu 24.04) from this tag with circuits aeacf31 beside it; git_sha 86941a1.
Check SHA256SUMS before installing.