fix(zsh): harden _radian_version_check against a contaminated version cache

[Data-Wise]

Summary

• _radian_version_check's R-vs-radian version-drift warning compared the cached version to the current R version with no validation of either side
• If the cache ever held radian's own version string (X.Y.Z shaped, e.g. 0.6.16 — indistinguishable from a real R version by a plain semver regex) instead of R's, the next login produced a nonsensical warning like R changed: 0.6.16 -> 4.6.1 even though nothing was actually wrong
• Fix requires major version >= 2 on both the freshly-read R version and the cached value before comparing — R has not shipped a major version below 2 in decades, radian has never left major version 0, so this is the actual discriminator (a plain X.Y.Z shape check is not, since radian's own version matches it too)
• A malformed or contaminated cached value is now silently ignored (and self-heals on the next write) instead of producing a false warning

Test plan

• zsh -n zsh/.zshrc — syntax clean
• E2E: extracted the live function, stubbed R, ran it against an isolated HOME/cache in 3 scenarios against the actual shipped code:
  • Contaminated cache (0.6.16 vs real 4.6.1) → no warning, cache self-heals to 4.6.1
  • Genuine version change (4.5.0 vs 4.6.1) → warns correctly
  • No change (4.6.1 vs 4.6.1) → silent
• First draft of the fix (plain ^[0-9]+\.[0-9]+\.[0-9]+$ regex) was caught by this same E2E process — it still fired on the contaminated-cache case since radian's version also matches that shape — corrected to the major->=2 check before landing

🤖 Generated with Claude Code