feat: add an explicit low-filament override

[voska]

An insufficient RFID filament estimate blocks a send even when the operator accepts swapping spools at runout. Add --allow-low-filament to preflight and print send to turn only that amount failure into an explicit warning. It works with --strict, applies to every mapped filament, and preserves the other gates and --confirm requirement.

JSON records preflight.allow_low_filament; confirmed sends warn on stderr even with --quiet. Document the flag in the README, specification, and changelog.

Validation: make ci passed, including race tests. Regression tests cover default refusal, overrides, other blocking gates, multiple slots, confirmation, and dry-run behavior. A read-only dry run of a real sliced job exits 9 without the flag and 0 with it, retaining the amount warning. Review gate: approved the CLI surface, output compatibility, and send/dry-run paths.