Cardano Updates

## Summary
- Add GitHub Spec Kit scaffold (`.specify/` directory,
`.claude/commands/` slash commands)
- Write project constitution capturing wallet's principles:
maintenance-first stability, era-aware design, type safety, formal
specification, reproducible builds, comprehensive testing, and code
quality gates
- Add `.gitignore` exception for `.specify/memory/` (blocked by existing
bare `memory` pattern)

## Test plan
- [ ] Verify `.specify/` directory structure is complete
- [ ] Verify constitution reflects project conventions
- [ ] Verify slash commands are available in Claude Code sessions

Add speckit scaffold (.specify/ directory, slash commands, constitution)
to enable the SDD workflow for future feature development.

Dependency versions are managed by nix (haskell.nix) and
cabal.project constraints. The .cabal bounds were redundant
and added maintenance cost during dependency bumps.

Closes #5177

## Summary

### Upload bug fix
The `upload()` function used `find "$dir" $pattern` where `$pattern` was
`-name '*.tar.gz'` with embedded quotes. Shell expansion turned the
quotes into literal characters, so `find` never matched `.tar.gz` or
`.zip` files. Only the docker image (`-type f`) uploaded correctly.
Fixed by passing the file extension directly.

### Artifact verification
Added a `verify-artifacts` job between `build-artifacts` and
`create-release`:
1. Downloads each platform artifact
2. Extracts the archive
3. Verifies the binary exists
4. Runs `cardano-wallet version` smoke test (Linux/macOS — skipped for
Windows cross-compiled binaries)

`create-release` now depends on `verify-artifacts`, so corrupted builds
can't ship.

### Other
- Bump `download-artifact` v4 → v8

## Test plan

- [x] CI passes
- [ ] Manual `workflow_dispatch` release test with `test` type

Closes #5223