Fix -n LIMIT restarting on every line

[VXNCXNX]

-n LIMIT is documented as "Limit the number of replacements that can occur per file", but in the default line-by-line mode the limit restarts on every line, so there is no overall cap. Only -A/--across honours the documented behaviour. On a file of three a a lines:

$ sd -n 2 --preview a X file.txt      $ sd -A -n 2 --preview a X file.txt
X X                                   X X
X X                                   a a
X X   <- 6 replacements for -n 2      a a   <- 2, as documented

process_reader_line_by_line called replacer.replace once per line and each call restarted from self.replacements. replacen now takes a &mut budget it decrements, and the reader creates one budget per input instead of one per line. Replacer::replace keeps its signature, so the across path and the library API are unchanged.

-n 2 now matches -A -n 2 exactly and unlimited output is untouched. Two tests added in sd-cli/tests/cli.rs. Suite passes, clippy clean. cargo fmt --check flags one pre-existing diff in that file, not in code I touched, so I left it.

AI disclosure: written with Claude Code. I ran the binary before and after and checked the mutation myself.