fix(queue): reject duplicate per-identity enrollment (Closes #99)

[Fabiana1504]

Closes #99.

Adds a per-identity duplicate guard to the queue contract so a single identity can no longer occupy multiple positions (the one-position-per-identity fairness guarantee).

Feature (issue #99)

• Per-identity index — storage key ("identity_pos", identity) -> u32 (the position id), mirroring the existing ("pos", id) tuple layout so it never collides with position or scalar keys.
• enroll_position() — checks the index before creating a position and panics with "identity_already_enrolled" on a duplicate; writes the index entry after the position is created.
• cancel_position() — removes the index entry, so a cancelled identity may re-enroll. total_enrolled is unaffected (it counts next_id - 1, including cancelled positions).
• get_position_by_identity(env, identity) -> Option<u32> — added to the Queue trait and implemented; O(1) lookup instead of unbounded iteration.
• Tests (4) — duplicate enroll panics; distinct identities both succeed; index cleared on cancel (+ re-enroll works); get_position_by_identity after enroll and after cancel.
• SDK — QueueClient.getPositionByIdentity() read method (mirrors getPosition).

Why this key format & concurrency: the ("identity_pos", identity) tuple matches the contract's existing ("pos", id) convention, namespacing the entry via a Symbol discriminator. Each enroll_position() is a full transaction; Soroban serializes transactions at the ledger level and storage writes are atomic within a transaction, so the check-then-write index update cannot race.

Pre-existing fixes (required to build/run the tests)

cargo test -p lineproof-queue did not compile on main (an earlier merge left the test target broken). These were unrelated to #99 but necessary to satisfy the "tests pass" criterion:

• test.rs: removed a duplicate use import and a duplicate name: struct field; changed |&id| closures to |id| (soroban Vec::iter() yields owned values).
• initialize(): removed an extend_ttl() on the contract address as a persistent key that never exists — it panicked with Storage/MissingValue and broke nearly every test.
• Made expire_position() / expire_positions_batch() pub so the generated client exposes them (their tests call client.expire_position(...)).
• Two expire tests used advance(&admin, &0) (a no-op that never transitions state), so expire panicked; switched them to close(), a valid state for expiry.

Testing

cargo test -p lineproof-queue → 29 passed; 0 failed. SDK queue.ts typechecks clean. (Auto-generated test_snapshots/ are gitignored, so not included.)

Closes #99.

Co-Authored-By: Claude Opus 4.8 noreply@anthropic.com

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