feat(highlight): add MdRenderInlineCode bg to distinguish code from headings

doc/md-render.jax

@@ -812,6 +812,10 @@ Markdown をレンダリングしてバッファに表示する完全な例で
 
   *Bold*                太字（フォールバック; `bold = true` ）
   *Italic*              斜体（フォールバック; `italic = true` ）
+  *MdRenderInlineCode*  インラインコード（ `` `code` `` および `<code>` ）。
+                        fg は `String` を継承し、bg は控えめなニュートラル色
+                        （Comment fg と Normal bg のブレンド）でコードと
+                        見出しを視覚的に区別する。
   *MdRenderHighlight*   Obsidian `==highlight==` マーカー
   *MdRenderTag*         Obsidian `#tag` （ `Label` にリンク）
   *MdRenderMath*        数式（ `Special` にリンク）

doc/md-render.txt

@@ -802,6 +802,10 @@ INLINE FORMATTING ~
 
   *Bold*                Bold text (fallback; `bold = true`)
   *Italic*              Italic text (fallback; `italic = true`)
+  *MdRenderInlineCode*  Inline code spans (`` `code` `` and `<code>`).
+                        fg follows `String`; bg is a subtle neutral tint
+                        (Comment fg blended with Normal bg) so code is
+                        visibly distinct from headings.
   *MdRenderHighlight*   Obsidian `==highlight==` markers
   *MdRenderTag*         Obsidian `#tag` syntax (links to `Label`)
   *MdRenderMath*        Inline/block math (links to `Special`)

lua/md-render/init.lua

@@ -97,6 +97,7 @@ function M.setup_highlights()
   M.setup_alert_highlights()
   M.setup_details_highlights()
   M.setup_image_placeholder_highlight()
+  M.setup_inline_code_highlight()
 end
 
 --- Set up image placeholder highlight group (MdRenderImagePlaceholder)
@@ -112,6 +113,27 @@ function M.setup_image_placeholder_highlight()
   vim.api.nvim_set_hl(0, "MdRenderImagePlaceholder", { fg = fg, bg = bg, default = true })
 end
 
+--- Set up inline code highlight group (MdRenderInlineCode).
+--- fg follows String so the existing color cue is preserved; bg is a subtle
+--- neutral tint (Comment fg blended with Normal bg) so backtick spans stay
+--- visually distinct from headings even when the colorscheme paints both green.
+function M.setup_inline_code_highlight()
+  local normal_hl = vim.api.nvim_get_hl(0, { name = "NormalFloat", link = false })
+  if not normal_hl.bg then
+    normal_hl = vim.api.nvim_get_hl(0, { name = "Normal", link = false })
+  end
+  local normal_bg = normal_hl.bg or 0x1e1e2e
+  local comment_hl = vim.api.nvim_get_hl(0, { name = "Comment", link = false })
+  local tint = comment_hl.fg or 0x888888
+  local bg = blend_color(tint, normal_bg, 0.18)
+  local string_hl = vim.api.nvim_get_hl(0, { name = "String", link = false })
+  if string_hl.fg then
+    vim.api.nvim_set_hl(0, "MdRenderInlineCode", { fg = string_hl.fg, bg = bg, default = true })
+  else
+    vim.api.nvim_set_hl(0, "MdRenderInlineCode", { bg = bg, default = true })
+  end
+end
+
 -- Re-export submodules (preview is lazy-loaded to avoid circular dependency)
 M.ContentBuilder = require("md-render.content_builder").ContentBuilder
 M.Markdown = require "md-render.markdown"

lua/md-render/markdown.lua

@@ -1057,7 +1057,7 @@ local HTML_TAG_HIGHLIGHTS = {
   strong = "Bold",
   i = "Italic",
   em = "Italic",
-  code = "String",
+  code = "MdRenderInlineCode",
   s = "DiagnosticDeprecated",
   del = "DiagnosticDeprecated",
   strike = "DiagnosticDeprecated",
@@ -1562,8 +1562,8 @@ Markdown.render = function(text, repo_base_url, autolinks, ref_links, footnote_m
   rendered_text = process_paired_markers(rendered_text, "==([^=]+)==", "MdRenderHighlight", 2, highlights, links)
   rendered_text = process_inline_math(rendered_text, "MdRenderMath", highlights, links)
 
-  -- Restore code spans (adds String highlight for each span)
-  rendered_text = restore_code_spans(rendered_text, code_spans, "String", highlights, links)
+  -- Restore code spans (adds MdRenderInlineCode highlight for each span)
+  rendered_text = restore_code_spans(rendered_text, code_spans, "MdRenderInlineCode", highlights, links)
 
   -- Restore backslash-escaped characters (adjusts highlight/link positions)
   rendered_text = restore_backslashes(rendered_text, backslash_escapes, highlights, links)

tests/markdown_checkbox_test.lua

@@ -451,10 +451,10 @@ test("html: <em> renders as Italic", function()
   assert_eq(highlights[1].hl, "Italic", "html em: should be Italic")
 end)
 
-test("html: <code> renders as String", function()
+test("html: <code> renders as MdRenderInlineCode", function()
   local text, highlights = render("<code>code</code>")
   assert_eq(text, "code", "html code: tags should be stripped")
-  assert_eq(highlights[1].hl, "String", "html code: should be String")
+  assert_eq(highlights[1].hl, "MdRenderInlineCode", "html code: should be MdRenderInlineCode")
 end)
 
 test("html: <s> renders as strikethrough", function()