fix: reject non-finite ranks when importing

[pcruz1905]

Importing a data file whose rank is inf, nan, or just larger than an
f64 can hold throws away the rest of the database:

$ cat ~/.z
/aaa|5|1000000000
/bbb|inf|1000000000
/ccc|3|1000000000
/ddd|10|1000000000

$ zoxide import z; echo "exit $?"
exit 0

$ zoxide query --list --score --all
   NaN /bbb

Three directories are gone and the command reports success.

Database::age scales every rank by 0.9 * max_age / total. One non-finite
rank makes total infinite, so the factor is 0.0: every other directory is
multiplied down to 0.0, drops below the 1.0 threshold and is removed,
while the entry responsible survives, because NaN < 1.0 is false like every
other comparison against it.

It does not stop there. The NaN keeps the total NaN, so total_age > max_age
is never true again and the database stops ageing permanently — 20 dirs at
rank 600 stay at a total of 12000 instead of being scaled back to 9000.

last_accessed is already validated, in that parse::<u64>() rejects
anything unreasonable and reports it with a line number. Doing the same for
the rank keeps the bad line out and leaves the rest of the file alone:

$ zoxide import z
/home/user/.z:2: invalid entry: /bbb|inf|1000000000

$ zoxide query --list --score --all
   1.2 /aaa
   0.8 /ccc

age drops non-finite ranks too, so a database that has already been hit by
this recovers on the next run rather than staying stuck.

This covers z, fasd, z.lua and zsh-z, which share the parser.
autojump passes its rank through a sigmoid and was never affected.

Testing

cargo test (26 tests), cargo +nightly fmt --check and
cargo clippy --all-targets -- -D warnings are clean.

Added a parametrised parser test covering inf, -inf, nan and 1e400
alongside the ranks that must keep working, including a path containing |,
and two age tests: one that the finite entries survive a poisoned database,
one that ageing still brings the total back under max_age. Reverting either
half of the fix on its own fails its tests and leaves the valid cases green.