Update from code changes: Cobalt B20 Asset scheduled multiplier reference

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Summary

Adds a Cobalt hardfork reference for the B20 Asset multiplier surface migrating to ERC-8056 ("Scaled UI Amount"), documenting the new scheduled updateUIMultiplier path and the Beryl → Cobalt compatibility promise.

Changes

• New page base-chain/specs/upgrades/cobalt/b20-asset-scheduled-multiplier.mdx covering audience + compatibility promise, an old → new symbol mapping (functions, events, errors with selectors/topic0s), the new updateUIMultiplier / toUIAmount / fromUIAmount / MAX_UI_MULTIPLIER / IScaledUIAmountConversion surface and lifecycle, and Q&A-style edge cases.
• Explicitly notes Cobalt is not yet live and every Cobalt-only selector is undialable until activation.
• Registers the page under the Cobalt group in docs.json.

Context

Mirrors the new per-hardfork changelog entry added in base/base-std#197 (docs(changelog): add changelog dir + Cobalt B20 Asset scheduled-multiplier entry).

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Project	Status	Preview	Updated (UTC)
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