fix(feeder): publish USD per USTC for MM2

[lunctoken]

Problem

The MM2 core interprets the Oracle-only UST meta-denom as USD per USTC. The feeder currently applies its legacy fiat/LUNC conversion to every denom, so UST is submitted as USTC per LUNC instead. Both components work independently, but their price contract is incompatible and produces incorrect MM2 swap quotes.

Design confirmation

StrathCole confirmed in this review comment that MM2 was designed with:

• UST as an Oracle-only meta-denom carrying USD per USTC;
• uusd as the unchanged on-chain bank denom;
• all legacy fiat/LUNC conversions unchanged.

Solution

• publish the raw USTC/USD value for the UST meta-denom;
• keep the existing fiat/LUNC conversion unchanged for every legacy Oracle denom;
• add unit and integration assertions that pin the special UST semantics.

The on-chain bank denom remains uusd; this change applies only to the Oracle meta-denom used by MM2.

Validation

• go test -count=1 ./pkg/feeder/voter passes, including the existing live Terra Classic RPC integration tests and dry-run prevote construction;
• go build ./... passes;
• the repository-wide short test suite is currently blocked by pre-existing pkg/feeder/keystore test compilation errors after the SDK signing API change; this PR does not modify that package;
• the corresponding core work is tracked in Market-Module-2-0/core#3;
• specification: MM2 no-mint proposal.

The price contract is now confirmed and the focused validation is complete, so this PR is ready for review.

[lunctoken]

@StrathCole, could you please confirm the Oracle price contract expected by MM2?

This draft assumes that:

• UST is an Oracle-only meta-denom carrying USD per USTC;
• uusd remains the on-chain bank denom;
• all legacy fiat/LUNC conversions remain unchanged.

The corresponding on-chain implementation and full validation report are in Market-Module-2-0/core#3. Both PRs are intentionally kept as drafts until this convention is confirmed and the remaining multi-validator tests are complete.

[StrathCole]

@StrathCole, could you please confirm the Oracle price contract expected by MM2?

This draft assumes that:

* `UST` is an Oracle-only meta-denom carrying USD per USTC;

* `uusd` remains the on-chain bank denom;

* all legacy fiat/LUNC conversions remain unchanged.

The corresponding on-chain implementation and full validation report are in Market-Module-2-0/core#3. Both PRs are intentionally kept as drafts until this convention is confirmed and the remaining multi-validator tests are complete.

Yes this is what the mm was designed like.