feat(buffer): add configurable eol_format for EOL virtual text

Problem: EOL virtual text order (category → recurrence → due) and the
double-space separator are hardcoded in `apply_extmarks()`. Users cannot
reorder, omit, or restyle metadata fields.

Solution: Add `eol_format` config field (default `'%c  %r  %d'`) with
`%c`, `%r`, `%d` specifiers. `parse_eol_format()` tokenizes the format
string; `build_eol_virt()` resolves specifiers against `LineMeta` and
collapses literals around absent fields.

lua/pending/config.lua

@@ -64,6 +64,7 @@
 ---@field keymaps pending.Keymaps
 ---@field folding? boolean|pending.FoldingConfig
 ---@field sync? pending.SyncConfig
+---@field eol_format? string
 ---@field icons pending.Icons
 
 ---@class pending.config
@@ -78,6 +79,7 @@ local defaults = {
   date_syntax = 'due',
   recur_syntax = 'rec',
   someday_date = '9999-12-30',
+  eol_format = '%c  %r  %d',
   folding = true,
   category_order = {},
   keymaps = {