refactor: code-golf pass + explanatory comments

Two logical changes that landed as one file rewrite:

1. Tighten the code. SLOC 168 -> 126 (-42), total lines 201 -> 146
   pre-comments. Moves: inline single-statement if/while bodies (via
   stylua's `collapse_simple_statement = Always` and `column_width
   = 120`), collapse K-termcode table to one line, combine api/fn/M
   into one local declaration, drop verbose config comments, merge
   `two` / `ivs_by_line` / `li` into one local, inline CR-branch
   loop, drop the apply-indirection in setup(), and assorted smaller
   trims.

2. Re-add prose where the golf made intent non-obvious. Comments now
   explain: smartcase detection, the +start_line/-2 offset reconcile,
   the *global* avail disqualification, the cursor-proximity sort
   purpose, the slow/fast char encoding of pair labels (and why it
   keeps `prefixes` minimal), the overlap-skip interval math and the
   carryover-to-next-match behavior, why the augroup is recreated per
   setup call, and why table-form labels own a dedicated highlight
   group. No code change from the comment pass itself.

Final sizes: 126 SLOC, 194 total lines including 55 comment lines
and 13 blank lines. Still smaller than upstream (131 SLOC) and one
to two orders of magnitude smaller than every other jump plugin.

stylua.toml

@@ -1,4 +1,5 @@
-column_width = 80
+column_width = 120
 indent_type = 'Spaces'
 indent_width = 2
 quote_style = 'AutoPreferSingle'
+collapse_simple_statement = 'Always'