docs: update spectrum-ts documentation for v12.1.0

[photon-action-bot]

Documentation Update: spectrum-ts v12.1.0

This PR updates documentation for spectrum-ts based on the v12.1.0 release.

Release Notes

Breaking Changes

• Platform IDs now use lowercase snake_case everywhere. Update checks like "iMessage", "Slack", "Terminal", and "WhatsApp Business" to "imessage", "slack", "terminal", and "whatsapp_business".
• Custom definePlatform() IDs must also be lowercase snake_case, like "my_platform". IDs with uppercase letters, spaces, hyphens, leading/trailing underscores, or double underscores now throw.
• @spectrum-ts/imessage-local now exports localIMessage instead of imessage, with platform ID "local_imessage".

New Features

• Cloud iMessage and local iMessage can now be registered in the same Spectrum() app. Use message.platform to distinguish "imessage" from "local_imessage".

Bug Fixes

• Spectrum() now rejects duplicate providers for the same platform before initialization, so ambiguous provider setups fail early with a clear error.
• Cloud token renewal is now shared and coalesced across providers, avoiding overlapping refreshes and making retry behavior more consistent.

Improvement

• Provider lookup and docs now consistently use canonical platform IDs, while helper lookup still accepts legacy spellings where it can. ✨

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Version: 12.1.0

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🤖 This PR was auto-generated by Claude Code via the update-docs action in buildspace.

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[github-actions]

Superseded by #125 which targets a newer version.