feat(serial): Linux sysfs USB health/topology enrichment (#1091)

[zackees]

Implements the Linux slice of #1091: sysfs-based USB health/topology enrichment, designed so every line of parsing logic is fixture-testable on any OS.

• Platform-neutral core: sysfs_usb parses a rooted directory tree; only live_root() (/sys/bus/usb/devices) is Linux-gated. Interfaces are detected structurally (subdirectory containing bInterfaceClass) instead of by Linux's colon-containing interface names — colons are illegal in NTFS filenames, so this keeps fixtures Windows-legal while matching real sysfs nesting.
• Conservative health mapping (per meta: Windows USB phantom-device detection and recoverable native-USB deployment #1149's locked invariants — facts preserved, Unknown never fabricated): PresentProblem only on concrete signals (authorized=0, unconfigured, CDC interface with no bound driver) using documented Linux-only problem codes explicitly unrelated to Windows CM_PROB_*; healthy requires a bound tty interface; everything ambiguous stays Unknown. The Windows UsbProblemDevice shape is not abused — Linux gets a distinct diagnostics-only LinuxUsbProblemDevice + present_usb_problem_devices_linux(); existing APIs behave identically on all platforms.
• behind_external_hub mirrors the Windows ancestry classification via port-path depth (1-1 root-attached vs 1-1.4 behind a hub).
• macOS: explicitly unchanged/Unknown — IOKit work requires a Mac host (documented in the module).

Validation (local)

• soldr cargo test -p fbuild-serial → 176 + 7 passed (15 new fixture tests: healthy CDC, hub vs root topology, unauthorized, unconfigured, driverless CDC interface, malformed/partial entries never panic, RP2040-typical synthetic VID/PID fixture, unowned tty stays Unknown); clippy -D warnings clean; dependents (fbuild-daemon, fbuild-cli) check clean; full bash test before merge.

Part of #1091 — remaining: macOS (IOKit) and the real-device validation runs on both platforms.

🤖 Generated with Claude Code

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📥 Commits

Reviewing files that changed from the base of the PR and between c7351bf and be818e0.

📒 Files selected for processing (4)

• crates/fbuild-serial/README.md
• crates/fbuild-serial/src/lib.rs
• crates/fbuild-serial/src/ports.rs
• crates/fbuild-serial/src/sysfs_usb.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/1091-linux-sysfs-usb

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