fix(precompiles): use checked_sub for total_supply in burn

[forumevi]

## Summary

In native_coin_authority.rs, the burn handler reads the current total_supply
from storage and then subtracts the burned amount using saturating_sub:

// before
&current_total_supply.saturating_sub(args.amount).to_be_bytes_vec()

The inline comment says "Underflow cannot happen due to the balance check", but
saturating_sub silently clamps to zero when an underflow does occur instead of
surfacing it as an error. This means that any bug or storage corruption that violates
the invariant (total_supply >= individual balance) would cause total_supply to
be written as 0 without any revert or observable signal — permanently corrupting
the global supply accounting.

## Fix

Replace saturating_sub with checked_sub and propagate a hard revert with
ERR_OVERFLOW on underflow, consistent with how mint already guards the
opposite direction:

// after
let new_total_supply = current_total_supply
    .checked_sub(args.amount)
    .ok_or_else(|| new_reverted_with_early_penalty(gas_counter, reservoir, ERR_OVERFLOW))?;

If the invariant holds (it always should on a correct chain), behaviour is
identical. If it ever does not hold, the transaction reverts loudly instead of
corrupting the supply silently.

## Tests

Two regression tests are added to native_coin_authority.rs:

• burn_decrements_total_supply_correctly — verifies the happy-path decrement is exact.
• burn_reverts_on_total_supply_underflow_instead_of_saturating — verifies that when total_supply is artificially set below the burn amount (simulating storage corruption), the call reverts with ERR_OVERFLOW rather than writing zero.

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