Function nsToMs

WebGPU timestamp queries resolve to nanoseconds (BigInt64Array values read back from the resolve buffer). The renderer's budgets and every other timing surface in this codebase (FrameStats.timestamp via performance.now()) are in milliseconds, so every raw pair gets converted through this one function.

Takes bigint (the actual readback type) so a caller cannot accidentally pass an already-lossy Number(timestamp) of a value that may exceed Number.MAX_SAFE_INTEGER (a query set can hold timestamps for a long-running session; ~104 days of nanoseconds overflows a JS number). The subtraction happens in bigint space and only the final, small (sub-second, in practice sub-100ms) millisecond duration is converted to number.

Clamps a negative delta (endNs < startNs) to 0 rather than returning a physically impossible negative duration. This is reachable: GPU timestamps are not guaranteed monotonic across a device reset (see this module's own doc above). 0 is the honest floor — a duration cannot be negative, and unlike a raw negative number it cannot silently drag a min/mean computed downstream into nonsense. This is the guard's one choke point: frameTotalMs/passDurationsMs (frame-timing.ts) sum this function's output directly, without ever routing it through computeDurationStats, so a guard placed only in computeDurationStats would miss that summation entirely. Use isNegativeDelta alongside this function where the caller wants to know a clamp happened.