fix(contracts): resolve remaining merge damage — full workspace green

[joelpeace48-cell]

Follow-up to the previous contracts-merge-corruption fix. That commit
resolved the unclosed-delimiter parse failures; this one resolves
everything the fuller cargo test --workspace / cargo clippy / wasm
build surfaced once the code could actually run.

campaign contract:

• do_register: the referral-count increment block had the exact same
  disease as the earlier do_register fix — a missing } after the
  if !already_attributed guard let its content bleed into an
  unconditional second increment further down, double-counting every
  referral. Closed the block, removed the duplicate, moved the
  (genuinely separate) referral-lock stamp after it.
• register(): the participation barrier (issue Contracts: Nullifier to prevent re-registration / double-claim after merkle root rotation #740) fired even when a
  participant was still currently active, turning ordinary idempotent
  re-registration calls into a hard error. Scoped the barrier to only
  apply when the participant is not currently registered, which is
  the scenario the barrier was actually meant to catch (re-registration
  after deregistration) — verified against Contracts: Nullifier to prevent re-registration / double-claim after merkle root rotation #740's own passing test,
  which only asserts on the deregistered-then-blocked case.
• test.rs: a 3-way function-name/body shuffle from the same merge
  scrambled test_clear_participation_allows_re_registration,
  test_referral_locked_allows_reregister_same_referrer, and
  test_referral_locked_prevents_referrer_switch_on_reregister —
  each name had a different test's body. Traced every affected function
  back to its origin commit (2efd49e, 15605f0, 976b2b9) and replaced
  the whole corrupted block wholesale with correctly-matched
  name/body pairs, fixing the argument order for the ones sourced from
  the pre-invite-code signature along the way.
• test_root_rotation_does_not_allow_double_registration had a second
  test's body pasted in front of its own content; removed the leaked
  block.
• One test had an admin-nonce off-by-one (&1 instead of &0 for the
  first post-initialize admin call).
• 8 tests whose scenario is deregister-then-reregister now correctly
  expect NullifierAlreadyUsed (the participation barrier, confirmed
  intentional and permanent) instead of the referral-loop/lock errors
  they predate — those code paths are provably unreachable via this
  vector now (a referral loop requires the referrer to already be
  registered when the cycle-closing registration is attempted, which
  requires them to have deregistered first, which the barrier now
  blocks before loop-detection is ever reached).
• fuzz_test.rs: one stray extra argument in two set_merkle_root calls,
  one clippy needless_range_loop.

rewards contract:

• Renumbered the governance/multisig error block (was 21-31, colliding
  three ways with the SEP-41 block, a simpler multisig scheme matching
  campaign contract's, and each other) to 36-46/47-51 — confirmed via
  docs/CONTRACTS_API.md that only the SEP-41 codes (21-29) are actually
  published to clients; the colliding ranges aren't documented anywhere
  and are free to move.
• Gated the Poseidon-hash Merkle experiment (poseidon.rs, merkle.rs)
  behind an off-by-default poseidon-experimental feature — it
  depends on ark_bn254 and light_poseidon, neither ever declared in
  Cargo.toml, so it has never compiled since the commit that added it.
  Nothing in the contract's public API calls into it.
• One clippy collapsible_if.

Workflows: contracts-ci.yml already had the fix for this — Soroban
dropped support for wasm32-unknown-unknown on Rust 1.82+, and
dtolnay/rust-toolchain@stable now installs whatever the latest stable
is. Applied the same wasm32v1-none target across every other workflow
and script that still referenced the old target (ci-performance.yml,
performance-gate.yml, testnet-integration.yml, docs.yml,
release-bindings.yml, package.json's build:contracts fallback) so none
of them silently break the next time stable Rust ships.

Verified end to end: cargo fmt --all -- --check, cargo clippy --workspace --all-targets -- -D warnings -A deprecated, cargo test --workspace (campaign 84/84, rewards 99/99, all others green), and
cargo build --target wasm32v1-none --release for both contracts all
pass clean.