Add a mangled-macro scan and a ratio inset for the OMA comparison

check_texhealth.py flags control characters and bare macro stubs left
by a shell heredoc, a class of corruption that LaTeX compiles without
complaint. It skips when main.tex is absent, as in this package.

fig_sec_snr now carries an inset with the OMA-to-proposed SER ratio,
since the 1 to 9 percent advantage is invisible across two decades of
log axis, and save() now guards inset overlap as it guards the legend.

check_consistency covers the L=8 crossover, the inset ratio span, the
stated secret sizes, and the Fig. 5 curve coincidence: 29 assertions.

code/check_texhealth.py

@@ -0,0 +1,49 @@
+# -*- coding: utf-8 -*-
+"""Guard against mangled TeX control sequences.
+
+Shell heredocs silently turn a backslash escape into the control
+character it names, so \times becomes a tab followed by "imes" and \ref
+becomes a carriage return followed by "ef". LaTeX compiles both without
+an error and prints the wreckage, so the build log cannot catch this.
+This scan can.
+"""
+import re
+import sys
+from pathlib import Path
+
+TEX = Path(__file__).resolve().parents[1] / "main.tex"
+CTRL = {"\t": "TAB", "\r": "CR", "\x08": "BS", "\x0c": "FF",
+        "\x07": "BEL", "\x0b": "VT", "\x00": "NUL"}
+# a macro name shorn of its first letter, which is what the escape ate
+STUBS = ["ef{", "abel{", "ite{", "extbf{", "extit{", "ext{", "imes",
+         "rac{", "eft(", "ight)", "ho_", "elta", "psilon", "ambda",
+         "igma", "ewline", "otag", "uad", "nderline", "ag{", "egin{",
+         "nd{", "aption{", "ilde{", "ar{", "at{", "ec{"]
+PAT = re.compile("(?<![" + chr(92)*2 + "A-Za-z0-9])(" +
+                 "|".join(re.escape(x) for x in STUBS) + ")")
+
+
+def main():
+    if not TEX.exists():
+        print("  SKIP  tex health :: main.tex not in this package")
+        return 0
+    lines = TEX.read_text(encoding="utf-8").split("\n")
+    hits = []
+    for i, line in enumerate(lines, 1):
+        for ch, name in CTRL.items():
+            if ch in line:
+                hits.append("CTRL %s line %d: %r" % (name, i, line[:90]))
+        for m in PAT.finditer(line):
+            seg = line[max(0, m.start() - 30):m.start() + 30]
+            hits.append("STUB %r line %d: %r" % (m.group(1), i, seg))
+    for h in hits:
+        print("  FAIL  " + h)
+    if hits:
+        print("tex health: %d suspicious sequences" % len(hits))
+        return 1
+    print("  PASS  tex health :: no mangled control sequences")
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())