agent: add adapter for Rakuten Ichiba

[alectimison-maker]

Summary

• add a Rakuten Ichiba adapter for the researched Japanese marketplace search, item, shop, and cart surfaces
• guide the agent around sponsored results, variant and subscription prices, shop-separated carts, shipping exceptions, and conditional points
• mirror the adapter in Chrome and Firefox with routing, spoofing-boundary, behavior, and parity tests

Motivation

CONTRIBUTING.md lists rakuten.co.jp as a high-priority East Asia adapter, but WebBrain currently uses only generic commerce behavior there. Current official search results and Rakuten help for multi-shop shipping, shop-grouped carts, and 39 Shop shipping exceptions expose several non-obvious traps:

• [PR] results are sponsored, while 〜 prices and variants make the first displayed price non-comparable
• 定期購入 is a recurring purchase with a different price from the normal option
• different shops have independent carts, orders, shipments, and shipping policies
• the 3,980円 free-shipping line is evaluated within an eligible shop/order and does not override 特定送料 or destination/delivery exceptions
• points and Super DEAL rewards are conditional and are not an immediate reduction of the payable price

Without site guidance, these behaviors can lead to incorrect cheapest-price claims, unintended subscriptions, combined-shipping promises, or overstated savings.

Design

The adapter explicitly matches only the researched Rakuten Ichiba surfaces:

• www.rakuten.co.jp
• search.rakuten.co.jp
• item.rakuten.co.jp
• basket.step.rakuten.co.jp

It deliberately excludes Rakuten Travel, Books, unresearched order/checkout hosts, other group services, and suffix/path lookalikes.

Six dated, imperative notes tell the agent to:

• distinguish add-to-cart and checkout labels
• ignore sponsored position and compare the same exact variant by price plus shipping
• select required options and avoid recurring purchase unless requested
• handle desktop/mobile shop-separated cart behavior accurately
• verify 39 Shop thresholds and shipping exceptions per shop
• report payable totals separately from conditional points

This follows the existing regional-adapter shape without adding APIs, permissions, dependencies, or network calls.

Testing

• production-module focused assertions for trusted/rejected URLs and Chrome/Firefox note parity — passed
• node --check src/chrome/src/agent/adapters.js — passed
• node --check src/firefox/src/agent/adapters.js — passed
• exact Chrome/Firefox Rakuten-block diff — passed
• npm run test:security — passed, 60/60 checks
• npm run test:webmcp — passed with real Chrome
• node test/run.js — 1,354 passed; one unchanged baseline failure remains because package.json is 26.0.1 while the newest CHANGELOG.md entry is 26.0.0

Compatibility and risks

This is additive prompt metadata with no public API or configuration changes. The main residual risk is UI drift; the notes are dated 2026-07, use stable marketplace labels and documented policies rather than selectors, and keep host matching conservative.

Scope

Rakuten Travel, Books, other group services, unresearched checkout hosts, and mobile-app-only flows are intentionally deferred.

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