fix: two-step fee recipient, checked fee arithmetic, slash floor, tok…

[Helix-xi]

fix: two-step fee recipient, checked fee arithmetic, slash floor & SEP-41
token probe

Closes #30
Closes #31
Closes #32
Closes #33

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Summary

This PR addresses four security and correctness issues identified in the
intent settlement contract.

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#30 — Two-step fee recipient confirmation

set_fee_recipient has been replaced with a propose_fee_recipient /
accept_fee_recipient two-step pattern, mirroring the existing
transfer_admin flow. The admin proposes a new address, and that address
must explicitly authorize the handover before it becomes active. A typo'd
or unreachable address can no longer silently misroute every future
protocol fee and slash payment.

#31 — Explicit checked arithmetic for protocol fee calculation

The fee calculation in fill_intent (fill_amount * PROTOCOL_FEE_BPS /
10_000) now uses checked_mul / checked_div with a dedicated FeeOverflow
error, making the overflow-safety property visible in the code itself
rather than relying solely on the overflow-checks = true release profile
setting in Cargo.toml.

#32 — Minimum slash amount floor

slash_solver now applies .max(1) to the computed slash_amount. Without
this, a bond_amount below 10 (in the token's smallest unit) would round to
a zero slash via integer division, letting a failed fill go economically
unpunished. A non-zero bond now always produces a non-zero slash.

#33 — SEP-41 interface validation in add_allowed_dst_token

Before storing an allowlist entry, add_allowed_dst_token now calls
decimals() on the candidate address as a lightweight SEP-41 interface
probe. If the address doesn't implement the token interface the call traps
and reverts the transaction, surfacing the error at admin time rather than
silently allowing a non-token that would only fail later inside
fill_intent's transfer call.

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Tests

• admin_can_transfer_admin updated to use the new propose/accept flow
• fill_intent_fee_overflow_returns_error — verifies FeeOverflow fires at
  i128::MAX / 5 + 1
• fill_intent_fee_at_boundary_does_not_overflow — sanity check at the
  exact boundary
• slash_tiny_bond_always_yields_nonzero_slash — uses direct storage
  injection to place a bond of 5, confirms slash ≥ 1
• add_allowed_dst_token_rejects_non_token_contract — passes a non-token
  contract address, asserts failure and no storage write
• add_allowed_dst_token_accepts_real_token — positive case confirming a
  real SAC passes the probe

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