Add dashed line support to BufferPolylineMaterial

[DanielLeone]

Adds dashes to BufferPolylineMaterial. Ticket:
FTP-16167. Targets this fork's main —
not proposed upstream.

The API

Property	Default	Meaning
gapColor	Color.TRANSPARENT	drawn in the gaps; transparent discards, so gaps are holes
dashRepeat	1	pattern cycles along the whole line, 0–255
dashOffset	0	phase within a cycle, 0–1
dashPattern	65535 (0xFFFF)	16-bit on/off bitmask sampled within each cycle

Defaults are fully solid, so existing polylines are untouched.

How it works

The material layout grows 12 bytes (GAP_COLOR_U32, DASH_REPEAT_U8,
DASH_OFFSET_U8, DASH_PATTERN_U16) with matching pack/unpack, and the values
reach the shaders through a new dashParams vec4 vertex attribute carrying
(encodedGapColorRGB8, gapAlpha, dashRepeat + dashOffset, dashPattern). The integer
cycle count and the fractional phase share one float; the shader splits them with
floor()/fract(). dashOffset is stored in 256ths so a quarter turn round-trips
exactly.

The fragment shader walks the pattern along texCoord (0..1 down the line) rather
than gl_FragCoord, so dashes are anchored to the geometry: bands stay put on
the line as the camera pans, and scale with the line as it zooms, instead of
crawling in screen space. That's the main design choice here and it's the opposite
of upstream's PolylineDashMaterialProperty, which is sized in screen pixels.

Checked

• eslint, prettier and tsc (the material file is // @ts-check) via the repo hooks
• wireless-manager-front-end builds and bundles clean with this branch linked in
  (bun run build, exit 0, no new warnings)

Not checked yet

• visual output — no screenshots of actual dashed lines yet
• cesium's own specs for the buffer-polyline path
• cost of the extra per-vertex vec4 on large collections, which is the thing most
  worth measuring before this goes near a customer site

Note on consuming it

Only main publishes to our registry, so the front-end can't pick this up as
@ftpsolutions/cesium@… until it merges. Until then, use bun run cesium:link in
the front-end against this branch.

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