A build swap still spends its session budget loading the directive vocabulary on the watcher thread

Workspace.markAllForRecalculation calls loadVocabulary before it enqueues anything, and that call is a read on the session-wide reader, so it is bounded only by DevMojo.SESSION_READ_BUDGET, 30 s. The diagnostics drain has since moved off the triggering thread, which was the larger half of the same problem and is now fixed; this call did not move with it. Every build swap therefore still occupies the dev loop’s watcher thread for as long as the vocabulary read takes, and the thread it occupies is the one the loop needs for the next swap. Filed out of the drain item’s In Review review as the residue its "What changes" did not cover, rather than widened into it after approval.

Two things make this worth measuring before it is worth fixing. The read may well be cheap, in which case the finding is a latent hazard rather than a live cost and the item is about where the ceiling sits rather than about a number a developer feels; nobody has timed it. And the read is a different read from the drain’s, on the same reader, so if it is not cheap the two now contend on one connection in a way they did not when both were inline on one thread.

The second half is a diagnosability point, and it is the sharper one. The dev loop’s trace splits a mutation into workspace.mutate and workspace.notify, and docs/architecture/how-to/dev-loop-internals.adoc now teaches a reader that notify should be trivially small because the listener is a flag-and-submit. This call sits outside both spans, so time spent here is attributed to no span at all: a contributor investigating a slow build swap reads a small mutate, a small notify and a small drain span, and the trace offers nowhere for the missing seconds to have gone. Whatever the remedy for the blocking is, the span coverage is worth closing on its own, because a trace that accounts for none of a cost is worse than one that names it.

Not in scope: the cost of the vocabulary read itself as a database question, which is the shape the diagnostics-drain budget-overrun item, since shipped, took for the drain’s statement and which the store-performance skill’s method covers if a measurement here turns out to want it.