refactor: enforce layer boundaries so adapters stop reaching into each other

[DLANSAMA]

Step 1 of 4 in a structural refactor. Draft on purpose — see sequencing below.

Why

protocols/, slicer/ and download/ were already separate directories. Nothing enforced the separation, and it had drifted:

slicer/output.py:17   from bambu_cli.protocols.ftps import _remove_partial_file

The OrcaSlicer runner imported a private Bambu FTPS helper to delete a partial file, so a change to printer transport code silently changed slicer behavior. Five more of the same shape, four of them hidden behind function-local imports.

The load-bearing change

scripts/check_layers.py, now blocking in the lint job. It ranks every module and rejects any upward import, plus any import between the three sibling adapters. Deferred (function-local) imports count — they break the import cycle, not the dependency.

70  cli.py (sys.exit lives here)          80  bambu.py (__main__ shim)
50  commands/ interactive/ tui/           45  job/     40  download/ setup_cmd/
35  printer.py                            30  protocols/ | slicer/ | printables/   <- must not import each other
25  cliparse.py                           20  utils config context netsafety ams
10  constants errors paths logging_utils argutils jsonio tlspin fsutil

Directories were never the mechanism. This is.

Changes

Fix	Effect
New fsutil.py (rank 10)	The four pure path/file helpers stranded in protocols/ftps and download/naming
New cliparse.py	The argparse tree; cli.py drops 728 -> 308 lines and stays the only module with sys.exit
camera.py -> protocols/camera.py	It is a TLS transport sharing tlspin
get_ftp(printer), monitor_status(args, printer)	Take the printer instead of reaching up for an ambient one
JobSteps	commands.cmd_job is the composition root; a missing step raises MissingJobStep rather than importing across a layer
DIRECT_CAMERA_MODELS -> constants.py	doctor and preflight both read it without an upward import

Behavior is unchanged. Two tests were injecting printers by patching bambu_cli.printer.get_printer — module-global patching the repo guidance forbids — and now inject directly.

Gates

1146 passed, 1 deselected     (baseline 1145; +1 is a new missing-step test)
coverage 86.2%               (baseline 84.6%)
ruff / ruff format / mypy / bandit / syntax / cli-help / ci-workflow    green
check_layers.py: Layer boundaries OK (1 allowlisted debt edge)

Deliberately not fixed

• context -> printer — RuntimeContext lazily builds a BambuPrinter. Allowlisted in the checker with a written reason; the real fix is a composition root installing a printer factory, which would have made this 39-file diff unreviewable.
• protocols/mqtt.py (~880 LOC) — same reasoning: not splitting a hotspot inside a boundary refactor.

Both are now recorded in AGENTS.md under known architecture debt instead of being quietly dropped.

Sequencing

Draft because #97 (feat/tui) should merge first. This branch moves camera.py, splits cli.py, and inverts the commands <-> interactive edge; tui/ imports interactive, commands and protocols, so whichever lands second resolves conflicts. Once #97 is in, I will rebase this onto main and mark it ready.