fix(routines): drop placeholder zero sets and '@+0' bodyweight suffix

[DTTerastar]

Summary

Two follow-up fitdown polish fixes to v1.2.0 that surfaced once the routines export was in everyday use. Fitdown's spec only covers ${reps}@${poundage} and Nx compression — every edge case below is our extension.

1. Placeholder zero sets render as bare names

An exercise added to a routine in the Liftoff app but never given target sets/reps arrives as one or more zero/zero placeholder sets. Before:

Machine Shoulder Press
0@0

Sci-fit Upper Body
0:00

After:

Machine Shoulder Press

Sci-fit Upper Body

The exercise stays in the rendered routine (so an LLM agent reading "what's in my Free Weights routine?" still sees it) but the bogus 0@0 / 0@+0 / 0:00 lines are dropped.

2. Bodyweight reps without added load drop the @+0 suffix

Scapular Pull Ups\n5@+0 is the system telling you "5 reps at +0 added weight." The +0 is meaningless — it's just 5 bodyweight reps. Same for AB (assisted) with zero assistance, and WR with zero weight. Before:

Scapular Pull Ups
3@+0
6@+0

Captain's Chair Leg Raise
2x5@+0

After:

Scapular Pull Ups
3
6

Captain's Chair Leg Raise
2x5

Sets with real added weight (e.g. Decline Crunch: 10@+15) are unchanged.

3. DRY the set-line switch

workouts.go was duplicating routines.go's fitdownSetLine (the WR/BR/AB/WD/DD/ND mapping). They now share it, which is also how (1) and (2) apply to both renderers in one place. Helper signature is (exTypes, inputOne, inputTwo json.Number) so it doesn't care whether the caller has a SetData or PresetSetData.

Test plan

• go vet ./... clean; go build clean; go test ./cmd/... 19/19 passing (added cases for empty-set suppression on WR + ND, and zero-weight simplification for WR / BR / AB).
• liftoff-export routines show "Valley Creek 1" against live account — Machine Shoulder Press renders as a bare name, no 0@0.
• liftoff-export routines list — Free Weights tail (the 11 placeholder exercises) shows as bare names, no 0@-0 / 0@+0 / 0:00.
• liftoff-export workouts list --since 1d — no regression; real numbers render unchanged.

Out of scope

• JSON output is unchanged — placeholder sets are still present in --format json (upstream-faithful). This polish is markdown-only.
• v1.2.0 has these issues. A patch release (v1.2.1) is the natural next step after this merges.

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