Generalize double-brace fix for all bibtex math expressions

Multi-character superscripts/subscripts like cm^{-1}, ^{238}U,
^{60}Fe, and MU_{69} all suffered from Pybtex stripping their
braces, causing MathJax to render them incorrectly (e.g. ^-1,
where only the minus sign is superscripted).

Doubled the braces for all ^, _, and \mathrm arguments in the
bibliography to ensure they survive Pybtex's LaTeX-to-text conversion
and render correctly in MyST/Jupyter Book.

Author: Antigravity CLI

book/references.bib

@@ -489,7 +489,7 @@ @article{Porco2006
 
 @article{Howett2011,
     author  = {Howett, C. J. A. and Spencer, J. R. and Pearl, J. and Segura, M.},
-    title   = {High heat flow from {Enceladus}' south polar region measured using 10--600 cm$^{-1}$ {Cassini}/{CIRS} data},
+    title   = {High heat flow from {Enceladus}' south polar region measured using 10--600 cm$^{{-1}}$ {Cassini}/{CIRS} data},
     journal = {Journal of Geophysical Research: Planets},
     volume  = {116},
     pages   = {E03003},
@@ -2924,7 +2924,7 @@ @article{Connelly2012
 
 @article{Amelin2010,
     author  = {Amelin, Yuri and Kaltenbach, Angela and Iizuka, Tsuyoshi and Stirling, Claudine H. and Ireland, Trevor R. and Petaev, Michail and Jacobsen, Stein B.},
-    title   = {{U--Pb} chronology of the solar system's oldest solids with variable {${}^{238}$U}/{${}^{235}$U}},
+    title   = {{U--Pb} chronology of the solar system's oldest solids with variable {${}^{{238}}$U}/{${}^{{235}}$U}},
     journal = {Earth and Planetary Science Letters},
     volume  = {300},
     pages   = {343--350},
@@ -3013,7 +3013,7 @@ @article{Bollard2017
 
 @article{Hevey2006,
     author  = {Hevey, Phelim J. and Sanders, Ian S.},
-    title   = {A model for planetesimal meltdown by ${}^{26}$Al and its implications for meteorite parent bodies},
+    title   = {A model for planetesimal meltdown by ${}^{{26}}$Al and its implications for meteorite parent bodies},
     journal = {Meteoritics and Planetary Science},
     volume  = {41},
     number  = {1},
@@ -3464,7 +3464,7 @@ @article{Canup2011
 
 @article{Stern2019,
     author  = {Stern, S. Alan and Weaver, Harold A. and Spencer, John R. and Olkin, Catherine B. and Gladstone, G. Randall and Grundy, William M. and Moore, Jeffrey M. and Cruikshank, Dale P. and Elliott, Heather A. and McKinnon, William B. and Parker, Joel Wm. and Verbiscer, Anne J. and Young, Leslie A. and Aguilar, David A. and Albers, Jason M. and Andert, Tracy and Andrews, James P. and Bagenal, Fran and Banks, Maria E. and Bauer, Brian A. and Bauman, Jeffrey A. and Bechtold, Karen E. and Beddingfield, Chloe B. and Behrooz, N. and Beisser, Kerri B. and Benecchi, Susan D. and Bernardoni, Eduard and Beyer, Ross A. and Bhaskaran, Shyam and Birath, Eli and Bird, Michael K. and Boone, Daniel R. and Bowman, Anthony F. and Bray, Veronica J. and Britt, Daniel T. and Brown, L. E. and Buckley, Mark R. and Buie, Marc W. and Buratti, Bonnie J. and Burke, Lewis M.},
-    title   = {Initial results from the {New Horizons} exploration of 2014 {MU}$_{69}$, a small {Kuiper} belt object},
+    title   = {Initial results from the {New Horizons} exploration of 2014 {MU}$_{{69}}$, a small {Kuiper} belt object},
     journal = {Science},
     volume  = {364},
     number  = {6441},
@@ -4937,7 +4937,7 @@ @article{Wolszczan1994
 
 @article{Brennecka2010,
     author  = {Brennecka, Gregory A. and Weyer, Stefan and Wadhwa, Meenakshi and Janney, Philip E. and Zipfel, Jutta and Anbar, Ariel D.},
-    title   = {{$^{238}$U/$^{235}$U} variations in meteorites: extant {$^{247}$Cm} and implications for {Pb}-{Pb} dating},
+    title   = {{$^{{238}}$U/$^{{235}}$U} variations in meteorites: extant {$^{{247}}$Cm} and implications for {Pb}-{Pb} dating},
     journal = {Science},
     volume  = {327},
     number  = {5964},
@@ -5625,7 +5625,7 @@ @article{CastilloRogez2009
 
 @article{Rugel2009,
     author  = {Rugel, G. and Faestermann, T. and Knie, K. and Korschinek, G. and Poutivtsev, M. and Schumann, D. and Kivel, N. and Günther-Leopold, I. and Weinreich, R. and Wohlmuther, M.},
-    title   = {New Measurement of the $^{60}\mathrm{Fe}$ Half-Life},
+    title   = {New Measurement of the $^{{60}}\mathrm{{Fe}}$ Half-Life},
     journal = {Physical Review Letters},
     volume  = {103},
     number  = {7},