--structure silently ignores a template and mirrors with the default layout

[xroche]

-N reads a glued value as a preset number and a detached one as a user template, and the alias table classed --structure as param, which always glues. --structure "%h%p/%n%q.%t" therefore became -N%h%p/%n%q.%t, which the engine never consumes: the token fell through to the URL list and the crawl ran with the default structure at exit 0. It is a new paramn class rather than a reuse of param1 because #1379 tried that, and it turned --structure=1 into a template literally named 1, collapsing the mirror onto one colliding filename, also at exit 0.

paramn glues what -N can actually read glued: on/off, and a digit run of at most nine digits whose remainder holds no path separator and no extension dot. Every preset and every cluster param used to glue still glues, so --structure=1L0 remains preset 1 plus -L0, while a template reaches the engine detached even when it opens with a digit. The short arm uses a stricter test, a bare digit run and nothing else, because -N 2col/%n.%t has to stay a template. Anything left over has to carry a %: a value without one maps every URL onto a single local name, so --structure=flat is a typo rather than a template and is now refused, along with OFF, none, default, yes and -1, which used to mirror the whole site as ./flat and ./flat-2. --user-structure still takes such a value verbatim.

The nine-digit bound exists because a longer run does not read portably. sscanf("%d") past INT_MAX is undefined and glibc answers differently per width: on LP64 the value truncates negative, which is "userdef" with an empty template and crashes url_savename, while on ILP32 scanf clamps to INT_MAX, a preset whose sub-layout flattens the whole mirror onto one file. optreal_find matches -N exactly, so the alias table never sees the glued spelling and the engine counts the digits itself, calling atoi only on a run short enough to fit. -N4294967295 and --structure=4294967295 both abort on master.

-N 1 is preset 1 now instead of a template named 1. That semantics change is deliberate and approved; a template named 1 has no practical use. --user-structure, the spelling that always worked, was documented nowhere and now reaches the help text, and from there the man page, plus guide.html and cmdguide.html.

Test 359 asserts the whole mirrored file set on disk for every spelling rather than just the wanted path, so a value that fell through to the URL list shows up as an extra file. Each assertion was proven red against a mutant of the guard it covers, including a first-character-only predicate, an unbounded digit run, and an engine still reading the run with sscanf. -N2147483647 is what pins that last one on a 64-bit host: it is in range for an int and out of range for -N, so it fails the same way the i386 leg did.

Three of the test's own assumptions were POSIX-only and each showed up on a different leg. It read -N past INT_MAX as portable, which i386 contradicts. It compared against a path a trailing-slash TMPDIR gave a // the engine normalizes away, which macOS contradicts, fixed in test-timeout.sh so every test benefits. And it built its two absolute-path expectations from the POSIX form, while url_savename maps what Windows forbids in a filename to _ on every platform, so a drive-letter TMPDIR reaches disk as D_; those now derive through the same substitution rather than a hardcoded D_.

Closes #1380

The same failure family on the param class, which this does not touch, is filed as #1426.