spec: add initia charge method draft

[tansawit]

What

• add draft-initia-charge-00.md defining the initia payment method with the charge intent for Initia Stack MsgSend transfers
• align the document with the current upstream RFC/xml2rfc-style method spec structure and the unified-family framing used in PR feat: add unified EVM charge payment method #213
• support any Cosmos SDK coin on Initia Stack chains via currency as the challenged denom and amount as the challenged integer amount
• require methodDetails.chainId as the chain selector, using the Initia registry mainnets/ tree for chain metadata and the concrete Initia mainnet chain.json / assetlist.json as examples
• support optional server-paid fees via methodDetails.feePayer and feePayerAddress
• keep the v1 profile intentionally narrow to type="transaction" and single-message MsgSend verification

Why

This adds a concrete Initia method that matches the current Initia registry layout and works across Initia Stack chains that share the same MsgSend-based payment shape. The Initia mainnet entry and iUSD remain the worked examples, but the method is not limited to that chain or asset.

Design Notes

• chainId is the required chain discriminator for the unified initia method
• the spec does not define a separate bech32Prefix field because that metadata is derivable from the selected chain entry in the Initia registry
• this PR is intentionally scoped to the Initia Stack, not to Cosmos chains generally

Verification

• scripts/lint_frontmatter.py passes in a temporary Python virtualenv with python-frontmatter installed
• full make lint / make check were not run in this environment because Docker was unavailable

AI Disclosure

This spec and PR text were drafted with AI assistance. All content was reviewed and edited for technical accuracy, RFC compliance, and alignment with STYLE.md and CONTRIBUTING.md.

[github-actions]

Spec Preview

Spec	Changed
draft-card-charge-00	Yes
draft-httpauth-payment-00	Yes
draft-initia-charge-00	-
draft-lightning-charge-00	Yes
draft-lightning-session-00	Yes
draft-payment-discovery-00	Yes
draft-payment-intent-charge-00	Yes
draft-payment-transport-mcp-00	Yes
draft-solana-charge-00	Yes
draft-stellar-charge-00	Yes
draft-stripe-charge-00	Yes
draft-tempo-charge-00	Yes
draft-tempo-session-00	Yes

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