feat(water): the pause lands on the layer the slicer will swap at

[danstiner]

Stacked on #70 (which is stacked on #69). Implements what that spec derives.

The bug

The export placed the water pause one layer above the color line; the preview drew it at the line itself. Both were guesses about a grid neither modeled.

• zLine + layerMm is right only when the line falls in the upper half of its layer, and a full layer late otherwise. Measured on a 0.25 mm first layer: it emits 6.250 where 6.100 was available, handing a whole layer of land to the water color.
• The preview's bare line is wrong the other way — it draws land green that the print buries in blue. 41.0% of Bora Bora's land at the old defaults.

The change

pauseZ    = top(W + 1)     export print_z — top of the first layer that may print land,
                           the earliest swap leaving every water column its own color
boundaryZ = slice(W + 1)   what the print looks like — that layer's slice plane, half a layer lower

W is the last layer the water's surface reaches. colorChanges now takes the resulting height outright (waterZ) rather than an offset from the line, because that height is the grid's to decide and the line knows nothing about the grid.

First layer height becomes a setting

None of this is computable without it, and the export cannot pin it — writing any key into the project config stub would replace the user's presets (see #70). It is optional in the hash and written only when it differs from the default, so existing links stay byte-identical and decode to the 0.2 they were always assumed to have. Payload: 104 chars at the default, 120 with a custom value.

Defaults move to 0.2 first / 0.1 layer

boundary − zLine is a sawtooth over (0, h]: it shrinks toward zero, then snaps back a full layer the moment the line crosses a slice plane. A whole-mm base lands on a layer top exactly when h divides base − fh, putting the offset at h/2 — not the smallest, but the furthest from that snap in both directions.

grid	offset at a 6 mm base	distance to the snap	Bora Bora boundary	land printing water-colored
0.2 / 0.15	0.125 = 0.83·h	0.17·h	+10.0 m	41.0%
0.2 / 0.10	0.050 = 0.50·h	0.50·h	+4.0 m	16.0%

At 0.15 a 0.025 mm drift in base or exaggeration moves the printed boundary by 10 m of ground. At 0.1 it does not.

Swept all 29 presets at both layer defaults

• waterAsLand crossing the 1% warning threshold: none
• groove share moving: none
• color line moving: 3, all polders — Zeeland −77.59→−52.59, New Orleans −86.19→−61.19, Dead Sea −522.25→−492.25. Their anchor is 2 lifts below the lowest land and a lift is layerMm/K, so a thinner layer needs proportionally less margin. None of their warnings change.

Deferred

water.js still approximates the boundary as one full layer for its lakes groove ceiling and the waterAsLand warning. That over-states it by up to half a layer, so it under-warns rather than over-grooves. Moving it changes warning output, not export geometry, and belongs in its own PR — as does the land-prints-in-the-water-color warning that Bora Bora's 16% currently triggers nothing for.

442 tests pass (7 new), tsc clean.

🤖 Generated with Claude Code

https://claude.ai/code/session_01YSKNezHjK4jsY8UoP2Bquw