test(facade): pin empty-input round-trip uniformly across all codecs (#70)

[nao1215]

Summary

Pins the empty-input round-trip contract uniformly across all 32 facade pairs in yabase/facade. Without this test, a future refactor could silently flip the rule for one codec; with it, that surfaces as a single-test diff. Closes #70. Companion to yabase#63 / #64 / #65 (which all addressed non-empty inputs).

Changes

• test/empty_input_round_trip_test.gleam (new): one test per facade pair, asserting:
  • For every total encode_*: decode_*(encode_*(<<>>)) == Ok(<<>>).
  • For the two Result-returning encoders (encode_z85, encode_rfc1924_base85): let assert Ok(encoded) = encode_*(<<>>) then the same round-trip on encoded.
  • For the RFC 4648 family (Base16, Base32, Base32hex, Base64, Base64 strict, Base64url, Base64 nopadding, Base64url nopadding): decode_*("") == Ok(<<>>) directly.
• CHANGELOG.md: notes the new test under [Unreleased].

Design Decisions

• Hand-picked exhaustive test instead of metamon forall_round_trip: metamon is not a dev-dep of yabase, and the empty case is a single fixed input — a forall over input length would just amplify the work for a constant whose answer the test is defining. Listing every facade pair makes adding a new codec obviously incomplete (the new pair has to be added here too).
• decode("") only pinned for the RFC 4648 family: non-RFC-4648 codecs (Bech32, Base58Check) have their own conventions for "" (no version byte, no checksum) and shouldn't be coerced to a uniform contract. The issue explicitly notes this split.
• No production-code change: every test passes against the current facade, so this PR is pure regression coverage. If a future change to a codec breaks one of these, the failing test name names the codec.

Limitations

• README / per-codec docstring updates suggested in the issue's third acceptance bullet are not added in this PR. The test is the load-bearing artefact; a follow-up doc sweep can reference this test as the empty-case authority.
• Test count grew by 32 cases (gleam test reports 747 vs. 715 before).

Closes #70

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Configuration used: defaults

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Run ID: 72e5a5f8-3c98-4fe9-b735-4f983278e01b

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Reviewing files that changed from the base of the PR and between 71bd8bc and afbce1b.

📒 Files selected for processing (2)

• CHANGELOG.md
• test/empty_input_round_trip_test.gleam

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch test/issue-70-empty-input-round-trip

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