Add macOS as a first-tier platform including CI support

[garybeihl]

Summary:

This commit adds macOS to the CI build matrix including all needed infrastructure to support macOS builds in CI.

Changes:

• Add macOS to the CI matrix in ci.yml
• Install libftdi and libusb dependencies using Homebrew on macOS runner
• Relocate Linux dependency installation to separate step for clarity
• Build Humility from source on macOS in the absence of pre-built binaries
• Enable Humility manifest tests on macOS
• Enable clippy checking on macOS
• Fix clippy install by explicitly specifying active toolchain
• Update README documenting macOS as first-tier platform
• Add macOS prerequisites section including Homebrew setup instructions

Testing:

Verified on macOS 15.6.1 (Intel x86_64):

• Built demo-stm32f4-discovery and demo-stm32h7-nucleo
• Passing all tests (cargo test --verbose --workspace)
• Passing all format checks (cargo fmt --all --check)
• Clippy checks are passing with no warnings

Related:

This addresses the request in README.mkdn for help maintaining macOS support and continuous builds.

[cbiffle]

Hello! I believe you're responding to the note in the README where we mentioned we're open to macOS CI if someone is willing to step up and maintain the platform. Since this is an unsolicited change with no prior discussion, can you tell us a bit about yourself and why you're interested in being responsible for maintaining this support long-term?

In the interest of transparency, I'm also a little concerned that parts of this diff look like they may have been generated by an LLM -- it's slightly duplicative and misses some opportunities for optimization in the Github Actions design. The maintainer of macOS CI would need to have a fairly deep understanding of Github Actions, it isn't a task that can be handled long-term by prompting. (If you wrote this by hand, forgive my sensitivity — my writing tends to be detected as AI output due to my use of bulleted lists and fancy dashes.)