--stability=rc is treated as dev #67

Description

--stability=rc (lower case, the spelling Composer and RoadRunner tags use) is not recognised: Stability only knows RC. The command prints Possibly invalid stability (--stability=rc) and then Stability::toInt() maps the unknown value to weight 0, so the minimum stability silently becomes dev and alpha/beta releases are accepted too.

Expected: rc and RC are the same stability, as in Composer; --stability=rc admits only RC and stable releases.

Steps to reproduce

1. With releases v3.5.0-rc.1, v3.5.0-beta.1 and v3.4.0, run rr get --stability=rc --filter=3.5.0-beta.1.
2. The beta is installed instead of failing the stability check; --stability=RC rejects it.

Additional context

Found by the acceptance tests in #66 (VersionSelectionTest::prefersReleaseCandidateOverOlderBeta, together with #58: lower-case -rc tags also sort below -beta ones). The release lookup may be replaced by DLoad in #49.