refactor: keep one full record per line

Each line of measurements/<emulator>.jsonl now holds the whole run:
every repetition, core logs, per-frame hashes and raw timings. The gzip
data files, the data schema and the split and join code are gone, and
core_probe --record appends one line. record_version moves to 3.

The seven records are rewritten once: each new line equals its old line
joined with its data file, and rebuilds the first-layout file its id
hashes.

.gitattributes

@@ -23,7 +23,5 @@ bios/** -text
 data/** -text
 
 # Measurement records are append-only: two branches that each add a line to
-# the same file merge by keeping both. A data file is bytes its line vouches
-# for by sha256.
+# the same file merge by keeping both.
 measurements/*.jsonl text eol=lf merge=union
-measurements/data/** -text