feat: add Remix Studio and Atlas tooling

[leoafarias]

Description

Adds Remix-owned Atlas tooling:

• remix_atlas provides reusable catalogs, scenarios and state forcing, grid/story viewers, golden metadata and comparison, and producer/portable parity.
• remix_capture provides strict capture packaging and validation, portable component v2, comparison and indexing, and local/public GitHub sources.
• Remix Studio provides a macOS review app for immutable captures without executing producer code.

Pins only the shared mix and mix_protocol primitives from the merged conceptadev/mix#984. No external mix_atlas, mix_atlas_capture, or Muse dependency remains. Established mix_atlas/* wire identifiers are preserved for compatibility.

Includes the compact Fortal Button reference workflow, exact package-drift verification, Remix/Fortal UI fixes, documentation, fixtures, widget coverage, and desktop goldens. The optional full component capture corpus remains outside this foundational change.

Validation

• fvm dart run melos run test:flutter --no-select passed: Remix 1,843; remix_atlas 21; remix_capture 63; demo 15; Studio 35, with 4 intentional opt-in live tests skipped.
• Analyzer passed for remix_atlas, remix_capture, demo, and Remix Studio.
• fvm dart run melos run atlas:fortal:check passed and verified a 71-file, 520,933-byte bundle.
• fvm flutter build macos --debug passed for Remix Studio.

Visual Evidence

Related Issues

• Supersedes conceptadev/mix#982.
• Shared protocol prerequisites landed in conceptadev/mix#984.

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Checklist

Note: Updating the pubspec.yaml and CHANGELOG.md is not required. These are handled automatically during the release process.

• My PR includes unit or integration tests for all changed/updated/fixed behaviors.
• I have updated or added relevant documentation (doc comments with ///).
• I am prepared to follow up on review comments in a timely manner.

Breaking Change

Does this PR require users of the package to manually update their code?

• Yes, this is a breaking change.
• No, this is not a breaking change.