feat: two-char jump labels for overflow matches

When there are more matches than the single-char label pool (>|L|,
i.e. > 26 with the default), issue two-char labels so matches past
the single-char ceiling are still reachable.

Tactics:

- Cap: 2 * |L| = 52 labels total in two-char mode. The full |L|^2 =
  676 space produces visually unusable density on packed buffers;
  52 is plenty once the cursor-proximity sort lands them near the
  cursor.

- Label order: first char varies slow (26 labels starting with 'f',
  next 26 starting with 'j') so only 'f' and 'j' become prefixes.
  The remaining 24 letters stay usable for pattern extension in
  two-char mode.

- Per-line readability spacing: a 2-char label requires one column
  of original text between itself and the previous 2-char label on
  the same line. Prevents visual bleed when matches are adjacent
  (e.g. '0' search on timestamp-heavy text). Single-char mode
  unchanged.

- No global mode flag: a 'prefixes' set, populated while assigning
  labels, tells the input loop when to buffer a second keystroke.
  The set resets each iteration with 'active'.

lua/tiny-jump/init.lua

@@ -67,11 +67,14 @@ function M.start()
   local info = fn.getwininfo(win)[1]
   local top = info.topline
   local lines = api.nvim_buf_get_lines(buf, top - 1, info.botline, true)
-  local chars, active = '', {}
+  local chars, active, prefixes = '', {}, {}
 
   while true do
     api.nvim_echo({ { '/' .. chars, '' } }, false, {})
     local ch = fn.getcharstr(-1)
+    if prefixes[ch] then
+      ch = ch .. fn.getcharstr(-1)
+    end
     local jump_to = active[ch]
 
     if ch == K.ESC then
@@ -97,7 +100,7 @@ function M.start()
       chars = chars .. ch
     end
 
-    active = {}
+    active, prefixes = {}, {}
     api.nvim_buf_clear_namespace(buf, NS, 0, -1)
 
     if #chars > 0 then
@@ -111,6 +114,9 @@ function M.start()
         end
         return math.abs(a.start_col - cc) < math.abs(b.start_col - cc)
       end)
+      local L = M.config.labels
+      local two = #matches > #L
+      local line_end = {}
       local li = 1
       for _, m in ipairs(matches) do
         vim.hl.range(
@@ -121,20 +127,34 @@ function M.start()
           { m.line, m.end_col },
           { priority = 200 }
         )
-        while
-          li <= #M.config.labels and not avail[M.config.labels:sub(li, li)]
-        do
-          li = li + 1
+        local label
+        if two then
+          if li <= 2 * #L then
+            local a = math.floor((li - 1) / #L) + 1
+            local b = (li - 1) % #L + 1
+            label = L:sub(a, a) .. L:sub(b, b)
+          end
+        else
+          while li <= #L and not avail[L:sub(li, li)] do
+            li = li + 1
+          end
+          if li <= #L then
+            label = L:sub(li, li)
+          end
         end
-        if li <= #M.config.labels then
-          local label = M.config.labels:sub(li, li)
-          li = li + 1
+        local prev = line_end[m.line]
+        if label and (not prev or m.start_col >= prev) then
+          if two then
+            prefixes[label:sub(1, 1)] = true
+          end
           active[label] = { m.line + 1, m.start_col }
           api.nvim_buf_set_extmark(buf, NS, m.line, m.start_col, {
             virt_text = { { label, M.config.label } },
             virt_text_pos = 'overlay',
             priority = 201,
           })
+          line_end[m.line] = m.start_col + #label + (two and 1 or 0)
+          li = li + 1
         end
       end
     end