Add capped, paginated verifier management

[holystrings]

Summary

Caps the Verifiers set with MAX_VERIFIERS and adds offset/limit
pagination, closing off unbounded storage growth from add_verifier
and unbounded read cost from get_verifiers.

Changes

• MAX_VERIFIERS: u32 = 50
• add_verifier rejects new entries at capacity with the new
  MarketplaceError::VerifierLimitReached
• New get_verifiers_page(offset: u32, limit: u32) -> VerifierPage
  returns a correct slice plus total and next_offset (None once
  exhausted)
• get_verifiers(env) -> Vec<Verifier> is left unchanged for
  source-compat — it's called internally by add_verifier,
  remove_verifier, and merchant verification, so this avoids
  touching those paths

Testing

• add_verifier_rejects_when_at_capacity — fills to MAX_VERIFIERS,
  confirms the next add is rejected
• get_verifiers_page_returns_correct_slice — asserts items, total,
  and next_offset across a multi-page walk
• get_verifiers_page_offset_beyond_total_returns_empty — boundary case
• cargo test (full existing suite) and cargo clippy --all-targets -- -D warnings pass

Acceptance criteria

• Cap enforced on add
• Paginated read returns correct slices
• Existing tests still pass

Closes #116

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