Restructure into disk/ and unify writer internals

[justanotherariel]

Description

Stacked on #82.

• Commit 1: the progress channel carries FlashProgress directly (percentage derived once in FlashProgress::new), a shared run_with_progress replaces four copies of the forwarding loop, and every platform gets the same write_image → write_and_verify → write_to_device → verify_write skeleton with matching names and order. Windows verify-phase failures are now tagged VerificationFailed like Linux; macOS/Windows gain the post-finish progress drain (final "Write complete"/"Verification complete" updates are no longer lost).
• Commit 2: devices and disk_writer merge into one disk module (disk/{os}/{device,writer}.rs) plus remaining test dedup — motion only, review with --color-moved.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Refactoring (no functional changes)
• CI/CD changes

Related Issues

Stacked on #82 (part of the #14 stack).

Testing

• Unit tests added/updated (duplicated suites consolidated; platform tests follow their code into disk/{os}/)
• E2E tests added/updated
• Manually tested on macOS
• Manually tested on Windows
• Manually tested on Linux

cargo test -p hai-core --features mock and cargo test -p hai-desktop pass on Linux, including the Linux writer's write/verify roundtrip tests through the new channel type.

Screenshots

N/A — no UI changes.

Checklist

• My code follows the project's code style
• I have run cargo fmt and npm run lint
• I have added tests for my changes
• All new and existing tests pass
• I have updated documentation if needed
• My commits follow conventional commit format

DO NOT MERGE. After reviewing, merge #14 to merge all directly