Resume-at-line can miss the active feed rate before the recovery Z move

[righteousgambit]

Summary

Resume-at-line can recover an older feed rate when the active modal F word appears by itself or in tightly packed G-code. The recovered feed is emitted before the recovery Z movement, so an incorrect retained value can affect that move.

On develop at 2ea66c10e96d88013a04441134d3d4f8e8c88510, _find_last_feed_rate() only extracts an F word from lines it first recognizes as G1, G2, or G3: feed-recovery implementation. The controller then emits that recovered feed before the Z recovery move: recovery sequencing.

F is modal independently of whether the same line contains a motion command; LinuxCNC lists it among the other modal codes: G-code quick reference.

Minimal reproduction

G1 X0 F100
F275
G0 X1
G1 X2

Resume at line 4.

Expected:

• Feed recovery returns 275.
• When a Z position is available, the preview emits buffer G1 F275 before its recovery Z move.

Actual:

• Feed recovery skips the standalone F275 and returns the older F100.

A tightly packed line such as X10Y10G1F425 is also missed by the current motion-prefix heuristic.

Impact

The effect is state-dependent. If the effective feed was set on a line the current scanner misses and the machine retains a different or unknown feed, the recovery Z move can use the wrong feed. If retained feed already matches, no behavior difference is expected. Resume should explicitly reconstruct the program's most recent modal F value.

Acceptance criteria

• Recover the most recent valid F word before the resume line, whether standalone, spaced, or tightly packed.
• Ignore F words in semicolon and parenthesized comments.
• Emit the recovered feed before the recovery Z move.
• Cover both standalone and tightly packed forms with regression tests.