cardano-testnet: declare supported cli/node versions via README + --version output

[Jimbo4350]

Summary

cardano-testnet currently has no user-facing declaration of which cardano-cli and cardano-node versions it supports. The only source of truth is cardano-testnet.cabal (lines 44–45) and users have no way to answer "which cli/node should I use with this cardano-testnet?" without reading cabal files.

Prior art

• [BUG] - cardano-testnet - genesis.json: renameFile:renamePath:rename: does not exist | ADD COMPATIBILITY MATRIX #5795 — explicitly titled "ADD COMPATIBILITY MATRIX", closed not-planned
• [BUG] - Compatibility matrix page not found #5753 — docs.cardano.org matrix page 404'd
• [INFO] Compatibility matrix for cardano-node v9.0.0 (Chang hard fork) #5903 — user asking what versions work with node 9.0.0

The existing matrix at https://docs.cardano.org/developer-resources/release-notes/comp-matrix/ covers cardano-node / db-sync / rosetta-java / graphql / wallet but does not include cardano-cli or cardano-api. The backing repo cardano-foundation/docs-cardano-org was archived 2025-07-22, so extending it is not a viable path.

Proposal

Two one-time changes. Zero per-release maintenance.

1. README "Supported versions" section

Add to cardano-testnet/README.md:

cardano-testnet is designed to run the cardano-cli and cardano-node that ship in the same release. Mixing binaries across releases is unsupported; use cli and node from the same release tarball as this cardano-testnet.

The cardano-api and cardano-cli versions this build was compiled against are printed by cardano-testnet --version.

One rule covers every version pair. Leans on the existing major-version compat guarantee from consensus/network for same-release binaries.

2. Extend cardano-testnet --version

Report the cardano-api and cardano-cli versions this build was compiled against, sourced from the VERSION_cardano_api and VERSION_cardano_cli CPP macros cabal auto-generates based on the resolved dependency versions, so the cabal file remains the sole source of truth.

Target output:

cardano-testnet X.Y.Z - linux-x86_64 - ghc-9.6
git rev <sha>
built against cardano-api X.Y.Z.W
built against cardano-cli X.Y.Z.W

Why not the alternatives

• docs.cardano.org matrix update — external dependency, repo archived, no update cadence we control.
• Matrix-generating GHA — higher upfront cost for value a single-sentence policy already captures.

Acceptance

• cardano-testnet/README.md has a "Supported versions" section with the single-release policy statement.
• cardano-testnet --version output includes built against cardano-api X.Y.Z.W and built against cardano-cli X.Y.Z.W, sourced from cabal at build time.
• [BUG] - cardano-testnet - genesis.json: renameFile:renamePath:rename: does not exist | ADD COMPATIBILITY MATRIX #5795 and [INFO] Compatibility matrix for cardano-node v9.0.0 (Chang hard fork) #5903 are linked to the new README section and closed as resolved.
• PR cardano-testnet: Add backwards-compatibility with some old cli/node versions #6543 either withdraws the backwards-compat mechanism or narrows to the forward-only fix against the current cli release.