fix: make calculate_fee_split read-only

[Akinsuyiphilip]

Closes #498

Summary

Make calculate_fee_split a genuinely read-only fee quote operation.

Problem

calculate_fee_split previously reused storage helpers intended for mutating
payment paths. Those helpers could refresh merchant and settlement-rule TTLs
and emit bootstrap_fallback whenever no explicit rule was configured.

Because the quote endpoint is publicly callable, an attacker could repeatedly
invoke it to:

• Keep merchant and rule entries alive indefinitely.
• Prevent natural storage eviction.
• Generate unbounded fallback events and indexing noise.

This made a read path observable as a storage mutation and created avoidable
resource costs for the contract and downstream consumers.

Changes

• Added a side-effect-free settlement-rule resolver for read paths.
• Updated calculate_fee_split to use the read-only resolver.
• Prevented fee quote reads from extending merchant or rule TTLs.
• Prevented fee quote reads from emitting bootstrap_fallback.
• Preserved TTL refreshes and fallback telemetry for state-changing payment
  operations.
• Added regression coverage for TTL neutrality and event neutrality.

Security and behavior

calculate_fee_split still validates the merchant and amount and returns the
same fee split, but it no longer writes persistent storage or emits events.
Callers can safely use it for off-chain quotes without keeping otherwise
expirable entries alive or spamming fallback telemetry.

Testing

Added/updated coverage for:

• calculate_fee_split_read_is_ttl_and_event_neutral
• Bootstrap fallback behavior on state-changing paths
• Existing settlement and event-topic regressions

Validation:

cargo test --workspace

4 common tests passed
59 governance tests passed
88 settlement tests passed
All doc-tests passed

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