Fail fast when requested R version is not installed

[t-kalinowski]

Summary

When an explicit R version is requested and no installed R matches, fail directly from the installed R list instead of querying rig available --json.

This keeps the missing-version path simpler and avoids an unnecessary external lookup that can obscure the actual error. It also makes short exact-version requests behave as series requests: == 4 means 4.*.*, and == 4.4 means 4.4.*, selecting the latest installed matching version rather than treating omitted components as 0.

User-facing changes

• ir run --r-version <version> now reports a missing installed R directly when no installed version matches.
• --r-version 4 and --r-version "== 4" select an installed 4.x, choosing the latest matching installed version.
• --r-version "== 4.4" selects an installed 4.4.x, choosing the latest matching patch when multiple patches are installed.
• Short series specs still succeed with an older matching installed version when that is the only matching installed R.
• If no installed matching series exists, the error remains direct and does not query rig available --json.
• exclude-newer behavior is unchanged and still uses R availability metadata where needed.

Internal changes

• Added helper setup for fake installed R binaries in rig_selection tests.
• Added named CLI tests for major-series and minor-series selection with:
  • multiple installed matching versions
  • only one installed matching version
  • installed versions from other series only
• Added CLI regression coverage proving the missing explicit version path does not call rig available --json.
• Added a small helper for deriving rig install hints from bare and == R version requirements.

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