Cardano Updates

Agent-Logs-Url: https://github.com/IntersectMBO/formal-ledger-specifications/sessions/48e75ad1-6d6f-45d2-bb7d-c88a90a2458a

Co-authored-by: williamdemeo <[email protected]>

Co-authored-by: Copilot Autofix powered by AI <[email protected]>

Now that the file lives at
`src-lib-exts/abstract-set-theory/Axiom/Set/Map/Extra.lagda.md`:

+ Update the top-level module declaration to
  `module abstract-set-theory.Axiom.Set.Map.Extra (th : Theory) where`,
  matching the rest of the abstract-set-theory subtree.
+ Replace `open import Ledger.Prelude` with explicit imports from
  `abstract-set-theory.Prelude` and stdlib modules. `src-lib-exts/`
  cannot depend on `Ledger.Prelude` (upward dependency).
+ Inline the five-line `_≢ᵐ_` definition that previously lived in
  the standalone `Extra.agda`, and delete `Extra.agda`.
+ Convert the file to literate-markdown formatting with brief
  section prose; the module body is otherwise unchanged.
+ Update the single downstream caller
  (`Ledger.Conway.Conformance.Equivalence.Deposits`) to import from
  the new location.

Closes #1194

Not strictly necessary, but it highlights a couple of important aspects;
e.g., how it relies on the left-bias of `∪ˡ` (which is why we don't need
the complement restriction... nice!)