Implement tile-join's --use-attribute-for-id

The option had been listed in tile-join's option table since 533e000 (#361)
removed the code behind it, so it parsed and then exited with "Unrecognized
option." #409 dropped the leftover entry rather than ship a usage message
advertising an option that didn't work. Implement it instead.

The ID handling that #361 removed only ever applied to attributes that came
back from a SQLite join, and that join went away in the same commit, so there
is no longer anywhere in this tool to hang it on. Instead the option takes the
ID from an attribute of either kind that tile-join does have: one already
present in the source tiles, or one joined from a CSV with -c, with a joined
value superseding a tile value of the same name just as it does for ordinary
attributes. The attribute is consumed rather than copied through, and it is
checked before -x, -y, -X, and --exclude-all-tile-attributes, so the ID can
come from an attribute that isn't kept -- which is the useful case, as in
`-x GEOID10 --use-attribute-for-id=GEOID10`.

The conversion from an attribute value to an ID, along with the warnings for
the values that can't be represented as one, moves out of serialize_feature()
into attribute_to_feature_id() in mvt.cpp, which both tools link, so that
tile-join reports the same problems the same way instead of silently
truncating. Its one difference is that tile-join converts stringified numbers
without any additional option, since it has no equivalent of tippecanoe's -aI.

Along the way, an attribute whose value is 0 is now usable as a feature ID.
The check that an ID survives a round trip through its string form compared
std::to_string() against the value with its leading zeros stripped, and
stripping the only digit of "0" left "" to compare against "0", so a zero was
rejected as too large to represent.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DRXWaDXVEZV4t1ddR6ZRcw

mvt.hpp

@@ -250,6 +250,7 @@ bool is_unsigned_integer(const char *s, unsigned long long *v);
 
 struct serial_val;
 serial_val mvt_value_to_serial_val(mvt_value const &v);
+bool attribute_to_feature_id(std::string const &key, serial_val const &val, bool convert_numeric, unsigned long long *id);
 
 void get_bbox(std::vector<mvt_geometry> const &geom,
 	      long long *xmin, long long *ymin, long long *xmax, long long *ymax,