Load the Changes diff quickly while the daemon reconciles worktrees (#9243)

* fix(server): keep cold diffs out from behind background git fan-out

Add a background lane to the git process scheduler and run reconciliation
checkout reads, observation-driven workspace refreshes and background fetches
in it. Bound reconciliation checkout reads to four at a time. Stop the Linux
file observer's startup scan from reporting existing files as creates, which
made every cold diff compute and discard its first revision.

* fix(server): pick the git job when its start token fires

Background jobs that already reserved a start-rate token no longer start
ahead of normal work submitted after them.

* fix(server): re-read the working tree once its watcher covers it

The Linux observer no longer reports pre-existing files at startup, so an
edit made between a workspace's first snapshot and its watcher subscribing
reached no consumer. WorkspaceGit now runs one background working-tree read
once observation setup completes with a live watcher.

Demand refreshes keep the caller's Git lane instead of forcing normal, so
user actions running at high priority are not demoted.

Scope the populated-tree observer test to Linux (FSEvents replays recent
writes) and update the observer benchmark test, which expected the removed
startup batch.

* fix(server): re-read checkout metadata once observation covers it

A branch switch made before the Git metadata watcher subscribed left the
cached branch stale. The post-coverage read now refreshes structure as well
as the working tree, matching the refresh the Linux startup events used to
trigger.

docs/data-model.md

@@ -340,6 +340,14 @@ the number of Git processes that have started but not exited. Every Git command
 including initial workspace reads, filesystem-triggered refreshes, background checks, and explicit
 requests.
 
+Commands wait in one of three lanes, drained in order: `high` (worktree creation and archive),
+`normal` (everything a caller waits for, including diffs and explicit refreshes), and `background`
+(workspace reconciliation, refreshes triggered by watchers, polls and fetches, and the fetches
+themselves). Daemon-initiated fan-out must run in the background lane: one rescan over ~90 worktrees
+queues over 1,000 commands, and in the normal lane every command a cold diff runs waited behind
+them. Set the lane on the runner with `createRunGitCommand(provenance, { priority })`, or around a
+whole operation with `runWithGitCommandPriority`.
+
 Environment variables override `config.json`:
 
 | Environment variable                 | Setting                  |