Preserve confirmed empty AMS spool weights

[BenJule]

Summary

Preserve a confirmed empty AMS spool as 0 g and 0% throughout device parsing, the web filament manager, and cloud serialization.

Root cause

A remaining weight of zero was treated as equivalent to an unknown value. Later fallbacks reconstructed the full initial weight and could persist an empty spool as full.

Changes

• preserve firmware-reported and calculated zero remaining weight
• distinguish unknown remaining weight from confirmed 0 g
• keep single-slot and batch payloads at 0 g and 0%
• serialize confirmed empty spools with netWeight: 0
• retain existing fallback behavior for genuinely unknown weight
• update regression and source-contract coverage

Validation

• buildSpoolFromTray.test.ts when local Node dependencies are available
• TypeScript project check when local Node dependencies are available
• git diff --check
• one GPG-signed commit

Full native validation remains delegated to Jenkins.

No local full build was performed.