K0 privacy package: eliminate ringsize-2 (wallet warning + min-ring-4 floor + NoSigner registry)

[liqdmetal]

Summary

The complete K0 privacy package, consolidated into one reviewable PR.

RingSize-2 transactions expose the signer on-chain by design — the ring is
sender + receiver, and parity selects the sender (Extract_signer). 56.8% of
mainnet txs use ringsize 2. This package eliminates the loophole in three
stages, sequenced so each is independently reviewable and the non-consensus
half ships immediately.

The fixes

Fix A — wallet warning (no hard fork, ships now)

The wallet/API half. When a transfer resolves to ringsize 2, the wallet sets a
PrivacyWarning surfaced through the RPC result. No consensus change.

Fix B1 — consensus min-ring-4 floor (hard fork)

From K0_MIN_RING4_HEIGHT, NORMAL/BURN txs with ringsize < 4 are rejected.
The floor of 4 = sender + receiver + 2 decoys is the smallest meaningful
anonymity set. Hardened against two bypasses found during wargaming:

• fork-boundary dodge — the floor keys off the chain tip, not the
  attacker-settable tx.Height (a tx can reference a block 11 back)
• SC_TX type-lie — a ringsize-2 NORMAL transfer stamped SC_TX (a no-op
  execution path) no longer dodges the gate

Fix B2 — uses_signer registry (hard fork)

RingSize-2 SC calls expose the signer the same way. The NoSigner bit (high
bit of the SC_META_DATA Type byte — 33-byte wire format unchanged) marks
contracts that never call SIGNER(), auto-detected at install via AST scan
(ContractUsesSigner). Existing contracts default to uses_signer=true
(behavior preserved). Then:

• SC_TX + ringsize 2 + NoSigner → rejected
• SC_INSTALL at ringsize < 4 → rejected

After B1+B2, ringsize 2 has no legitimate remaining use.

Why this is one PR now

Previously split across four PRs (#80, #82, #91, #98), each carrying a
4,400-file vendored-dependency diff that made review impractical. This is the
same code consolidated into 16 files / ~1,000 lines, with no vendor noise,
plus the build-manifest fix (upstream's go.mod is 3 lines with no
vendor/modules.txt, so fresh clones don't build).

Tests (22, all green)

• blockchain: k0_ringsize (floor reject / not-active / pre-fork), wargame
  bypass + SC_TX type-lie, k0_b2 consensus
• dvm: ContractUsesSigner (detect / no-false-positive / case-insensitive),
  SC_META NoSigner bit, Private+NoSigner coexist
• walletapi: ringsize-2 warning set, ring-4 no-warning, Transfer result field

Sequencing note

The fourth stage — Fix C, owner auth via verify_sig so gated contracts can
run at ringsize ≥ 4 — depends on the DVM v9 verify_sig intrinsic and remains
a separate PR (submitted separately against the intrinsics).

Supersedes

Closes the fragmented #80 (Fix A), #82 (Fix B1), #91 (Fix B2) in favour of
this single package. (#98 / Fix C stays open as it targets a different base.)

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Branch fix/k0-privacy-package, based on community-dev.