ci(remix): validate hosted Mix resolution

[leoafarias]

Description

Removes the demo's unused Git Mix and mix_protocol pins so workspace tests
resolve Remix's declared hosted Mix range. CI now prints that resolution and
rejects overrides or out-of-range versions before running the existing suite.

Validation: fvm dart run melos ci (2,091 tests), plus
fvm flutter analyze packages/remix and packages/demo.

Related Issues

Related to #91. Stacked on #80.

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Checklist

• My PR includes tests for all changed behavior.
• Documentation is not required for this CI-only change.
• I am prepared to follow up on review comments in a timely manner.

Breaking Change

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

[leoafarias]

Coordination update: absorbed into canonical #80

The hosted Mix consumer validation is now included directly in #80 as
e9ab45e. It removes the demo's local Mix override and verifies that consumers
resolve hosted mix 2.2.0-beta.1 against the declared constraint.

The checker and the full combined CI suite pass; all 2,104 tests, analysis,
formatting, clean generation, parity, documentation, and drift checks are
green.

No separate agent action remains. Closing this draft without merge because its
work is already in #80. Issue #91 remains open only to track delivery through
the canonical PR.