Rename repo to match new naming convention

Following the same pattern as the recent renames:

rainlanguage/rain.interpreter → rainlanguage/rainlang (publishes on Soldeer as rainlang)
rainlanguage/rain.raindex.interface → rainlanguage/raindex.interface (publishes as raindex-interface)

This repo should similarly drop the rain. prefix and use the modern interface suffix style. Proposed:

rainlanguage/rain.interpreter.interface → rainlanguage/interpreter.interface
Soldeer registry name: interpreter-interface

Touchpoints once renamed:

Republish to Soldeer under the new name (next minor; e.g. interpreter-interface@0.2.0).
Update consumers: rainlang, raindex.interface, raindex, rain.deploy, downstream contracts.
Versioned imports inside interpreter-interface itself stay correct (versioned soldeer paths in src/).

Done on the repo side, and the follow-ups are now tracked per-repo (no tracking issue needed — the producer cron will pick them up).

Repo: renamed rain.interpreter.interface → rainlang.interface; contents matched in #116; rainlang-interface@0.2.0 published to Soldeer.

Consumer migrations (each: migrate the rain-interpreter-interface soldeer dep → rainlang-interface@0.2.0):

(The real consumer set — rain.deploy doesn't pin this dep; rain.pyth / rain.erc4626.words / rain.merkle / rain.flare do.)

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