[0.1.7] Throttle gem-not-installed balloon to once per 15 min

[unurgunite]

Description

Throttles the "Docscribe gem not found" balloon so it no longer spams on every opened file.

showGemNotInstalledBalloon fired from doAnnotate on each .rb file when the gem is missing. Now the first show goes through immediately and repeats are suppressed per project directory until 15 minutes elapse (BALLOON_THROTTLE_MS). Other failures keep flowing to idea.log unchanged.

Changes

• annotator/DocscribeAnnotator.kt — lastGemBalloonShown map (keyed by projectDir) + pure shouldShowGemBalloon(lastShownMs, nowMs) gate checked before notify.
• annotator/DocscribeAnnotatorBalloonThrottleTest.kt — 5 tests: interval constant, first show, suppressed within window, allowed at boundary and after window.

Checklist

• Tests pass (./gradlew test)
• Formatting clean (./gradlew spotlessCheck)
• Static analysis clean (./gradlew detekt)
• Plugin verified (./gradlew verifyPlugin)
• CHANGELOG updated (if applicable)