fitInspectionPayload counted the UTF-8 size of an 8-12 MB wire with a
per-character loop and ran every omission step on the main thread in one
turn (PR #277 final review M7). The size is now counted by TextEncoder
encodeInto through one bounded scratch buffer, and each re-measure
follows a yieldControl turn, with an abort signal checked between steps.
The fit is async; the fitted payload is the same as before.