Zed's hang telemetry could say a hang happened, but not what the app was
doing. This PR adds a foreground journal to gpui (behind the `profiler`
feature): the main thread records task polls, action handlers, input
dispatches, window draws, and frame presentations into a fixed-size
ring, and boundary entries — a newly drawn frame's presentation, or the
foreground going idle — partition that stream into activity intervals.
Recording is designed to be cheap enough to ship enabled: sub-threshold
task polls fold into a counter instead of individual entries, and the
ring is drained in bounded chunks off the main thread.

A `HangDetector` drains the journal and reports an incident for any
interval containing a single event at or above the hang threshold (100ms
in release), or whose cumulative foreground spend reaches a frame budget
(8ms in release) — catching both one long stall and many small pieces of
work that together drop a frame. Incidents serialize with phase
(startup/steady), stall and active durations, busy fraction,
dirty-to-present time, and up to 8 contributors in start order with
nesting depth: task polls carry their spawn location, actions their
names, inputs their kind. Zed batches the top 10 incidents by stall per
30 minutes plus a total incident count into a "Hang Incidents" telemetry
event, flushes the remainder on quit, and drops the old per-location
"Hang Report" histograms this replaces.

Release Notes:

- N/A