Refactor G-Code converters to a stream-based interface

Replace the batch-oriented AbstractConverter / AbstractBatchConverter /
Pipeline / SingleConverter hierarchy with a unified streaming model:
StreamConverter exposes push/flush/reset, and StreamPipeline composes
stages with a cascading flush so late-bound items propagate down the
chain. All existing converters (ArcsToLines, ApplyHeightmap,
FeedRateConverter, FusionRestoreRapidMovements, ModifyFeedRate,
MovePath, ShakingGCode, StripComments) are reworked to the new API,
and the source program stays read-only.

Also gate the file-open action through UiPermissions in FrmMain so it
respects the current state behaviour.

Author: etet100