fix(highlight): use multiline extmark for hl_eol with start-position clearing

Problem: single-row extmarks (`end_row = buf_line, end_col = len`) do
not trigger `hl_eol` — Neovim requires `end_row > start_row`. Line
backgrounds stopped at the last character instead of extending to the
window edge.

Solution: use `end_row = buf_line + 1, end_col = 0` for line bg
extmarks. Replace per-hunk `nvim_buf_clear_namespace` with
`clear_ns_by_start` that queries extmarks by start position only,
so adjacent hunks' trailing `end_row` is never killed.

lua/diffs/highlight.lua

@@ -636,10 +636,9 @@ function M.highlight_hunk(bufnr, ns, hunk, opts)
       end
 
       if opts.highlights.background and is_diff_line then
-        local bg_end_col = raw_len or (line_len + qw)
         pcall(vim.api.nvim_buf_set_extmark, bufnr, ns, buf_line, 0, {
-          end_row = buf_line,
+          end_row = buf_line + 1,
-          end_col = bg_end_col,
+          end_col = 0,
           hl_group = line_hl,
           hl_eol = true,
           priority = p.line_bg,

lua/diffs/init.lua

@@ -437,7 +437,10 @@ local function ensure_cache(bufnr)
   if config.highlights.context.enabled then
     compute_hunk_context(hunks, config.highlights.context.lines)
   end
-  local carried = entry and not entry.pending_clear and carry_forward_highlighted(entry, hunks)
+  local carried = entry
+    and not entry.pending_clear
+    and #entry.hunks == #hunks
+    and carry_forward_highlighted(entry, hunks)
   hunk_cache[bufnr] = {
     hunks = hunks,
     tick = tick,
@@ -514,6 +517,18 @@ local function find_visible_hunks(hunks, toprow, botrow)
   return first, last
 end
 
+---@param bufnr integer
+---@param ns_id integer
+---@param start_row integer
+---@param end_row integer
+local function clear_ns_by_start(bufnr, ns_id, start_row, end_row)
+  local marks =
+    vim.api.nvim_buf_get_extmarks(bufnr, ns_id, { start_row, 0 }, { end_row - 1, 2147483647 }, {})
+  for _, m in ipairs(marks) do
+    vim.api.nvim_buf_del_extmark(bufnr, ns_id, m[1])
+  end
+end
+
 local function compute_highlight_groups(is_default)
   local normal = vim.api.nvim_get_hl(0, { name = 'Normal' })
   local diff_add = vim.api.nvim_get_hl(0, { name = 'DiffAdd' })
@@ -945,7 +960,7 @@ local function init()
           if hunk.header_start_line then
             clear_start = hunk.header_start_line - 1
           end
-          vim.api.nvim_buf_clear_namespace(bufnr, ns, clear_start, clear_end)
+          clear_ns_by_start(bufnr, ns, clear_start, clear_end)
           highlight.highlight_hunk(bufnr, ns, hunk, fast_hl_opts)
           entry.highlighted[i] = true
           count = count + 1
@@ -1185,6 +1200,7 @@ M._test = {
   invalidate_cache = invalidate_cache,
   hunks_eq = hunks_eq,
   process_pending_clear = process_pending_clear,
+  clear_ns_by_start = clear_ns_by_start,
   ft_retry_pending = ft_retry_pending,
   compute_hunk_context = compute_hunk_context,
   compute_highlight_groups = compute_highlight_groups,

spec/highlight_spec.lua

@@ -397,7 +397,7 @@ describe('highlight', function()
       delete_buffer(bufnr)
     end)
 
-    it('line bg extmark survives adjacent clear_namespace starting at next row', function()
+    it('nvim_buf_clear_namespace kills line bg extmark whose end_row bleeds into cleared range', function()
       local bufnr = create_buffer({
         'diff --git a/foo.py b/foo.py',
         '@@ -1,2 +1,2 @@',
@@ -423,6 +423,50 @@ describe('highlight', function()
       local last_body_row = hunk.start_line + #hunk.lines - 1
       vim.api.nvim_buf_clear_namespace(bufnr, ns, last_body_row + 1, last_body_row + 10)
 
+      local marks = vim.api.nvim_buf_get_extmarks(
+        bufnr,
+        ns,
+        { last_body_row, 0 },
+        { last_body_row, -1 },
+        { details = true }
+      )
+      local has_line_bg = false
+      for _, mark in ipairs(marks) do
+        if mark[4] and mark[4].hl_group == 'DiffsAdd' then
+          has_line_bg = true
+        end
+      end
+      assert.is_false(has_line_bg)
+      delete_buffer(bufnr)
+    end)
+
+    it('clear_ns_by_start preserves line bg extmark whose end_row bleeds past cleared range', function()
+      local bufnr = create_buffer({
+        'diff --git a/foo.py b/foo.py',
+        '@@ -1,2 +1,2 @@',
+        '-old',
+        '+new',
+      })
+
+      local hunk = {
+        filename = 'foo.py',
+        header_start_line = 1,
+        start_line = 2,
+        lines = { '-old', '+new' },
+        prefix_width = 1,
+      }
+
+      highlight.highlight_hunk(
+        bufnr,
+        ns,
+        hunk,
+        default_opts({ highlights = { background = true, treesitter = { enabled = false } } })
+      )
+
+      local last_body_row = hunk.start_line + #hunk.lines - 1
+      local clear_ns_by_start = require('diffs')._test.clear_ns_by_start
+      clear_ns_by_start(bufnr, ns, last_body_row + 1, last_body_row + 10)
+
       local marks = vim.api.nvim_buf_get_extmarks(
         bufnr,
         ns,