fix: embed firefox native host for browser setup

[ahamed40444]

Fixes #140.

Summary:

• Adds embedded Firefox native messaging host mode to jcode.
• Makes jcode browser setup create firefox-agent-bridge-host.exe from the running jcode binary when no upstream
  host asset exists.
• Registers the generated native host path instead of failing on Windows.
• Rejects stale manifests that point at browser.exe.
• Adds focused tests for host path/materialization behavior.

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