Feature/start print from z

[sstefanov]

Description

Adds a START_PRINT_FROM_Z feature that lets a user resume a print at a chosen Z height — useful as a manual fallback when no power-loss recovery file was saved (crash, manual abort, swapped SD card, etc.).

How it works (high level)

1. Selecting a file on the LCD opens a small action submenu:
   • Cancel
   • Print from Z: <value> (0 = normal print)
   • Print
2. When a non-zero target Z is chosen, the SD file is pre-scanned (no motion) to simulate G-code state (position, temps, fans, tool, relative/absolute modes, G92 offsets).
3. Scanning tracks the most recent line that established the current Z. Resuming triggers only when an extrusion move occurs at or beyond the target Z, avoiding false triggers from parking/purge moves.
4. A self-contained resume sequence is enqueued (heat bed/hotends, home XY, move to Z, restore extruder counter, restore fan speeds, continue file from the saved byte offset). No POWER_LOSS_RECOVERY dependency.
5. Live progress is reported during long scans: LCD status "Scan Z 1.2/12.0 56%" and serial "SkipToZ: 56% z=1.2/12.0 sdpos=12345".

New G-code: M1005 — internal entry point that triggers the scan and resume. Uses a free M-code slot. Documented in gcode.h.

New files

• Marlin/src/feature/skip_to_z.{h,cpp} — line-buffered scan engine (128 B buffer), lightweight parameter parser, idle() called every 100 ms to keep watchdog/thermals/UI alive on slow hardware.
• Marlin/src/gcode/feature/skipz/M1005.cpp — G-code entry point.

Modified files

• Configuration_adv.h — new option block under HAS_MEDIA (disabled by default) with START_PRINT_FROM_Z_MAX and START_PRINT_FROM_Z_DEBUG.
• gcode.{h,cpp} — declare and dispatch M1005.
• ini/features.ini — build_src_filter so skip_to_z.cpp and M1005.cpp are only compiled when the feature is enabled.
• sd/cardreader.cpp — openAndPrintFile() injects the alt sequence when SkipToZ::target_z > 0; one-shot (cleared after use).
• inc/SanityCheck.h — requires HAS_MEDIA only.
• lcd/menu/menu_media.cpp — per-file action submenu (menu_sd_file_action()), float52 edit item for target Z.
• lcd/menu/menu.h — current_edit_value() public static accessor on MenuEditItemBase so context-sensitive input can detect the active field.
• lcd/language/language_en.h — MSG_START_PRINT_FROM_Z, MSG_SKIP_TO_Z_SCAN.

ADC keypad (ZONESTAR_LCD etc.) — while editing target_z with Z homed, the RIGHT button loads the current Z position instead of clearing to zero. All other fields retain existing behavior.

Requirements

• SDSUPPORT

Benefits

• Lets users recover interrupted prints without a power-loss recovery file.
• Fully self-contained: no PLR flash cost on boards that don't need it.
• Smart extrusion-aware trigger avoids false resumes on parking/purge moves.
• Live scan progress keeps the user informed on slow hardware.
• Per-file action submenu keeps the file list uncluttered and intent explicit.

Configurations

Enable in Configuration_adv.h:

#define START_PRINT_FROM_Z
//#define START_PRINT_FROM_Z_MAX 300  // optional: clamp max Z value
//#define START_PRINT_FROM_Z_DEBUG    // optional: verbose serial output

No special hardware required beyond an SD card reader and an LCD.

Related Issues

Feature request — manual Z-skip resume for interrupted prints.