refactor(math): redefine world coordinates to zoom-16 pre-scaled space

[bhousel]

This PR redefines the "world" coordinate system that we added #297 to pre-scale the coordinate up to z16.

Background

Previously, wgs84ToWorld() produced coordinates in the 0..256 range (zoom 0). Converting world → screen then required:

scale = 2^z
screenX = (worldX - 128) * scale + translateX

This means every cached world coordinate must be multiplied by 2^z on every frame, at every zoom level.

What this changes

World coordinates are now pre-scaled to zoom 16, giving a range of 0..16,777,216 (= 256 × 2^16). The transform becomes:

scale = 2^(z - 16)
screenX = (worldX - 8_388_608) * scale + translateX

Key property: at z16 the scale is exactly 1.0 — world coordinates are pixel-perfect screen coordinates with no transform needed.

Motivation

Editors like Rapid project GeoJSON geometry once and cache the result as world coordinates, then render that cache every frame. With z0 world coords, this cached geometry must still be reprojected into screen coordinates for rendering.

By pre-scaling to z16 (much closer to the zooms that we actually the geometry at),

• World -> screen can simply be performed as a multiply or GPU transform
• Preserves a useful amount of floating point precision (i.e. details specified in pixels like dash lines or line widths are not infinitesimally small).
• Can avoid recomputing and storing another bunch of coordinate arrays and other supporting data on every change in zoom.
• This is especially helpful for Pixi.js pipelines where geometry is uploaded to the GPU once.

API compatibility

project() / unproject() compose wgs84ToWorld + worldToScreen (and inverses). These methods remain numerically invariant — all callers of project/unproject continue to work unchanged.

Tile.tileExtent values are now in z16 world space (0..16,777,216) instead of z0 (0..256).