Closes #48 Implemented: replace silent underflow clamp with typed panic in release_milestone_multi_asset asset_raised decrement

[Promise278]

Fixes an inconsistent underflow handling pattern in
release_milestone_multi_asset where a silent .unwrap_or(0).max(0)
clamp on the asset_raised decrement could mask a real ledger
invariant violation — while the surrounding code in the same
function correctly panics with a typed error on underflow.

Problem

In multi_asset_release.rs lines 188-189, the per-asset raised
counter is decremented using a silent clamp:

let new_asset_raised = asset_raised
.checked_sub(clamped_release)
.unwrap_or(0) // silent on underflow
.max(0); // second clamp, same intent

This is inconsistent with the typed-error contract established
by the release_amount path just above it in the same function:

let milestone_release = milestone.target_amount
.checked_sub(milestone.released_amount)
.unwrap_or_else(|| panic_with_error!(env, Error::MilestoneReleasedExceedsTarget));

One underflow path panics loudly with a typed error.
The other collapses silently to 0.

What Changed

campaign/src/multi_asset_release.rs

• Replaced .unwrap_or(0).max(0) with a typed panic
  that mirrors the rest of the function:

  let new_asset_raised = asset_raised
  .checked_sub(clamped_release)
  .unwrap_or_else(|| panic_with_error!(env, Error::LedgerUnderflow));

• Reconciled storage_set_total_raised in the surrounding
  loop with the same typed-panic approach for consistency

campaign/src/types.rs

• Added new Error::LedgerUnderflow variant (= 81,
  following the existing numbered scheme) so the cause
  is unmistakable in logs and distinguishable from
  the generic Error::Overflow attribution

campaign/src/test/release_milestone_tests.rs

• Added test that forces the underflow condition by
  injecting funds above the recorded asset_raised ledger
• Asserts that the typed LedgerUnderflow panic fires
  instead of silently collapsing to 0

Invariant Being Enforced

asset_raised >= clamped_release must always hold
by construction before the decrement runs.

If it does not hold, the contract has encountered
a state it cannot safely reason about. Surfacing it
loudly as a typed panic is correct — silent correction
via clamp is not.

How to Verify

cargo test

[Alqku]

Typed panic on underflow is the right move for #48, but two things: (1) the two new files campaign/src/test/scratch_test.rs and campaign/src/test_token_scratch.rs look like leftover debugging scratchpads — please drop those before we merge; (2) the new Error::LedgerUnderflow = 81 variant is unnecessary scope creep — Error::Overflow already covers arithmetic failure modes and keeps the error enum lean. Once those are cleaned up this is good to go.

[Alqku]

Hey @Promise278 👋 — the typed Error::LedgerUnderflow change on #48 looks great. However no CI checks have run on this PR. Could you rebase against the latest main (or push a no-op commit) so the workflow reruns? Once fmt/clippy/test/wasm are all green I'll merge ✅