Enforce ABI-version compatibility between aggregator and its target shards (#432)

[soloking1412]

Closes #432

Problem

ledgerlens-aggregator registered ledgerlens-score shards via add_shard with no check that a candidate shard actually implements the interface version the aggregator expects. A shard whose interface had drifted (older build, changed signature) would be accepted silently, surfacing later as failed or subtly incorrect cross-contract calls instead of a clear error at registration time.

Changes

• add_shard now verifies compatibility before accepting a shard. It calls the candidate's supports_interface for each capability the aggregator invokes on shards — score (get_score), gate (query_risk_gate), and aggr (get_aggregate_score) — and rejects the shard unless all are supported. The call is made defensively via try_supports_interface, so a shard lacking the function (an older/drifted build) traps gracefully and is treated as incompatible rather than aborting the registration transaction.
• Clear, dedicated error. Rejections return Error::IncompatibleInterface. Because the #[contracterror] enum is at the hard 50-variant XDR limit, this follows the crate's existing alias pattern.
• Capability detection over version comparison, consistent with the versioning policy in docs/interface-spec.md §3: an additive shard upgrade that keeps the three required capabilities still registers cleanly.
• Documentation. New §7 "Aggregator compatibility contract" in docs/interface-spec.md defines the targeted interface version, the required capabilities, the enforcement behavior, and the rationale.

Tests

Added to contracts/ledgerlens-aggregator/src/test.rs:

• accepts a compatible LedgerLensScoreContract
• rejects a shard advertising no capabilities
• rejects a shard missing only aggr
• rejects a legacy shard that lacks supports_interface entirely

Verification

• Aggregator: 8/8 tests pass (4 new)
• ledgerlens-score: 879 passed, 6 ignored, 0 failed
• cargo build --workspace and clippy clean