C8 provenance lane render contract

Status: design contract for ctxsafety/visual (#1225). This is a render-level requirement spec, not a new renderer.

Every rendered primitive that carries a number or point must carry exactly one provenance_lane: WITNESSED, OBSERVED, or MODELED. The lane is a visible channel, not metadata hidden in a tooltip. The renderer must encode the lane in the primitive itself and in the legend.

Lane definitions

Lane	Data source	Required channel
WITNESSED	fak-authored facts: max|Δ|=0 bit-exact comparisons and reuse-bit facts from fak’s own cache path.	Green #1a7f37, solid fill, legend label starts with WITNESSED.
OBSERVED	External telemetry relayed by fak, such as provider cache_read.	Blue #0969da, diagonal hatch, legend label starts with OBSERVED.
MODELED	Projected page-down-tier values and other estimates until measured on-device.	Amber #bf8700, crosshatch, legend label starts with MODELED.

Color and hatch are both load-bearing. A grayscale print, color-blind view, or low-contrast embed must still distinguish the three lanes.

Non-blending rule

No primitive may render a MODELED point identically to a WITNESSED point. A series must not be promoted from MODELED to WITNESSED by sharing a color, hatch, legend entry, or lane key with witnessed data.

Do not sum, average, stack, or smooth across provenance lanes into one rendered series. If a chart needs all three values at the same x coordinate, render three separate primitives or three faceted lanes. The same rule applies to segments: a provider cache_read segment is OBSERVED; fak’s reuse-bit segment is WITNESSED; a projected page-down-tier segment is MODELED.

Required honesty gate

The paired honesty test is TestModeledPrimitiveCannotRenderOnWitnessedLane in internal/cachewitness. It constructs a projected page-down-tier primitive with MODELED provenance and refuses it on the WITNESSED lane. The adjacent style test asserts that WITNESSED, OBSERVED, and MODELED have distinct visible encodings, so a modeled series cannot be emitted with the witnessed style by accident.