feat: add Base block explorer metadata

[findolor]

Why

The website needs chain-owned explorer metadata from the registry instead of a hard-coded Base explorer map.

What changed

• add block-explorer: https://basescan.org to the Base network settings
• pin the registry settings entry to the immutable settings commit containing that metadata

Validation

• parsed public and private generated settings as YAML
• decoded the public registry data URI and verified the embedded explorer field
• built a private artifact with a dummy private Base RPC and verified both the injected RPC and explorer metadata survive encoding
• confirmed the private embedded settings match the generated inspection file

Dependencies

• spec: docs: specify optional network block explorer rainlanguage/specs#54
• SDK parser: feat: add network block explorer settings rainlanguage/raindex#2833

Rollout note

The hosted private registry artifact still needs to be rebuilt and uploaded after merge; this PR only changes source-controlled registry content.

[findolor]

• feat: add Base block explorer metadata #55 👈 (View in Graphite)
• main

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