[Feature Request] Dumbbell inventory: progression and deload snap to the weights you own #376

What problem does this solve?

I train at home with adjustable dumbbells (turn the handle to change the weight). They only come in these weights, in kg:

3, 6, 8, 9, 11, 13, 15, 17, 18, 19, 21, 24

The steps are irregular (3, 2, 1, 2, 2, 2, 2, 1, 1, 2, 3), so no per-exercise inc fits them. Fixed-weight dumbbell racks at home and in many small gyms have the same problem.

What happens today (v1.3.9):

• Progression adds inc to an off-grid weight without snapping (addStep, Exercise increment step rounds #175). With inc: 2, 9 → 11 → … → 21 works, but the next step is 23 and I only have 24.
• Deload candidates always come from multiples of inc (positiveGridAround in selectDeloadCandidate). With inc: 2 it suggests 10, 12, 14, 16, 20 or 22, none of which I own.
• Below 9 kg (3 → 6 → 8 → 9) no step size works, which affects the light accessory lifts (lateral raises, curls, rear delts).
• Tapping +/- from an even weight snaps to the even grid as well.

So almost every deload, and every lift at the edges of the range, needs a manual correction during the workout.

The plate inventory (lib/plates.js) already solves this for barbells, but it only changes the display, and loadKind is 'none' for dumbbells.

Proposed approach

A dumbbell inventory next to the plate inventory, stored per unit and stamped with _ts the same way S.plates is:

S.dumbbells = { kg: { weights: [3, 6, 8, 9, 11, 13, 15, 17, 18, 19, 21, 24], _ts } }

When a profile has one, exercises whose equipment is dumbbell (or that loadKind marks as such) use the list instead of the inc grid:

• Progression "up": the next weight in the list above the current one, not current + inc.
• Deload: selectDeloadCandidate takes its candidate weights from the list (the nearest ones around ideal) instead of positiveGridAround. The Epley scoring stays the same.
• Stepper +/-: moves to the neighbouring weight in the list.
• Edges: at the top of the list, progression keeps adding reps (or holds and says the heaviest dumbbell is reached) instead of suggesting a weight that doesn't exist.
• Off-list current weight (history, import): treat it like an off-grid weight today. The next step is the nearest list weight in the direction of the change.

Without an inventory nothing changes: the inc grid stays the default, so existing profiles behave the same.

If it fits better into the v2 engine (#186), I'm happy to look at it there instead. A sorted list of loadable weights per equipment type might generalise better (it would also cover kettlebells and pin-loaded machines).

Dependencies

• This doesn't require adding a new dependency