Make the acceptance page answer the operator, not the timer

Presses on Start and Continue were being swallowed. Every poll rebuilt
the whole test table and the prompt buttons with innerHTML, and a button
destroyed between mousedown and mouseup raises no click event at all:
the press simply vanished. Measured on the page as it stood, a Start
button node was replaced 13 times in 40 seconds, and with a poll landing
mid-press 8 presses out of 8 were lost. Rows, prompt buttons and tool
entries are now built once and afterwards only updated in place through
setters that skip the write when the value has not changed; under the
same test 8 presses out of 8 land.

The page also felt slow because each poll re-read and re-parsed the whole
result log and recomputed all 42 domain fingerprints. A result record
carries its run log, so the file reaches megabytes over a campaign and
the poll cost grew with it. The log now parses each line once and reads
only the bytes appended since, and a fingerprint is memoized against the
manifest's content hash. On the same manifest, catalog and log, one
/state goes from 8.89 ms to 0.10 ms at 0.89 MB and from 41.79 ms to
0.23 ms at 10.62 MB, and no longer grows with the log.

Actions now report on the press instead of on the next poll: Start greys
every Start and marks the row, Continue and Abort grey themselves, and
each pulls the next poll forward. /state carries an ETag so an idle page
polls for a 304, the poll ticks faster during a run, and the connection
is kept alive rather than handshaking per request.

Keeping the connection alive exposed a hazard worth naming: a POST
refused before its body was read left that body in the socket, where the
next read took it for a request line. A refusal now ends the connection.

forgetest is a dev-only component and can never appear in a coverage map,
so this has no acceptance catalog consequence; the host tests carry the
proof, including one that fails if a per-poll innerHTML rebuild ever
comes back.

forgetest/forgetest/catalog.py

@@ -39,6 +39,7 @@ class Test:
         self.description = description or (fn.__doc__ or "").strip()
         self.fn = fn
         self._source_sha = None
+        self._fp = (None, None)   # (manifest content sha, fingerprint)
 
     @property
     def source_sha(self):
@@ -51,7 +52,17 @@ class Test:
         return self._source_sha
 
     def fingerprint(self, manifest):
-        return _manifest.fingerprint(manifest, self.covers, extra=[self.source_sha])
+        """The domain fingerprint on this manifest, memoized by the
+        manifest's content hash: the page recomputes every test's
+        fingerprint on every poll, and a manifest never changes under a
+        running tool."""
+        key = manifest.content_sha
+        if key and self._fp[0] == key:
+            return self._fp[1]
+        fp = _manifest.fingerprint(manifest, self.covers, extra=[self.source_sha])
+        if key:
+            self._fp = (key, fp)
+        return fp
 
     def definition(self):
         """The gate-visible definition (no implementation, no prose)."""