fix(telemetry): a surface that stopped recording is not one that never ran (#3720)

`out["sources"]` was built only from the windowed aggregate, so a source
with rows in `retrieval_logs` but none inside the window got no bucket at
all. Absent is exactly how a source that never existed renders, so a
surface that WAS recording and went silent became unreadable — #2663 one
level up, the failure that looks like the correct answer.

Two queries at different scopes, and only one shaped the output.
`_complete_from` reads all-time and knows every source the table has ever
held; the windowed loop dropped whatever it did not return.

Every such source now gets a zero bucket. Zero is a real measurement here
rather than a manufactured one: the all-time query proves the source was
recording, and it made no calls across a window it fully covers. No
`covers_window` special case is needed either — a source whose first row
fell after `since` would have that row IN the window and already hold a
bucket, so anything reaching this branch began before it.

The counts are 0 and everything else is null. A sampled distribution is
not the same claim as a call count, and rendering p50 as 0.0 for a source
nobody sampled would assert a measurement — #3311's mistake, in the
readout built to prevent it.

Found while fixing #3712's fixture, which failed with KeyError for this
exact reason.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011cPyzNnegXHr5iRMzzy5KJ

src/scribe/mcp/tools/search.py

@@ -262,6 +262,13 @@ It is an UPPER BOUND per surface: a pull records the door it came
     "no measurement" is not "partial measurement", the same distinction
     `suppression`'s null carries a few paragraphs up.
 
+    A SOURCE SHOWING `calls: 0` WAS RECORDING AND MADE NO CALLS. `sources`
+    lists every source the table has ever held, not only those active in the
+    window, so a surface that stopped firing stays visible rather than
+    disappearing — being absent is reserved for a source that has never
+    recorded at all. Its score fields are null, not zero: the calls are a
+    real observation, the distribution is not one.
+
     `rule_usage_failed: true` means that read failed while the rest of the
     readout stood. The counts are still present so a caller can render, but
     they are zeros meaning "could not find out", not "nothing happened" — do