fix(presets): correct error parsers for real compiler output

Problem: all three built-in error parsers were broken against real
compiler output. Typst set source to the relative file path, overriding
the provider name. LaTeX errors go to stdout but the parser only
received stderr. Pandoc's pattern matched "Error at" but not the real
"Error parsing YAML metadata at" format, and single-line parsing missed
multiline messages.

Solution: pass combined stdout+stderr to error_parser so LaTeX stdout
errors are visible. Remove source = file from the Typst parser so
diagnostic.lua defaults it to the provider name. Rewrite the Pandoc
parser with line-based lookahead: match (line N, column N) regardless
of prefix text, skip YAML parse exception lines when looking ahead for
the human-readable message. Rename stderr param to output throughout
diagnostic.lua, presets.lua, and init.lua annotations.

lua/preview/presets.lua

@@ -1,11 +1,11 @@
 local M = {}
 
----@param stderr string
+---@param output string
 ---@return preview.Diagnostic[]
-local function parse_typst(stderr)
+local function parse_typst(output)
   local diagnostics = {}
-  for line in stderr:gmatch('[^\r\n]+') do
-    local file, lnum, col, severity, msg = line:match('^(.+):(%d+):(%d+): (%w+): (.+)$')
+  for line in output:gmatch('[^\r\n]+') do
+    local _, lnum, col, severity, msg = line:match('^(.+):(%d+):(%d+): (%w+): (.+)$')
     if lnum then
       local sev = vim.diagnostic.severity.ERROR
       if severity == 'warning' then
@@ -16,18 +16,17 @@ local function parse_typst(stderr)
         col = tonumber(col) - 1,
         message = msg,
         severity = sev,
-        source = file,
       })
     end
   end
   return diagnostics
 end
 
----@param stderr string
+---@param output string
 ---@return preview.Diagnostic[]
-local function parse_latexmk(stderr)
+local function parse_latexmk(output)
   local diagnostics = {}
-  for line in stderr:gmatch('[^\r\n]+') do
+  for line in output:gmatch('[^\r\n]+') do
     local _, lnum, msg = line:match('^%.?/?(.+%.tex):(%d+): (.+)$')
     if lnum then
       table.insert(diagnostics, {
@@ -51,41 +50,45 @@ local function parse_latexmk(stderr)
   return diagnostics
 end
 
----@param stderr string
+---@param output string
 ---@return preview.Diagnostic[]
-local function parse_pandoc(stderr)
+local function parse_pandoc(output)
   local diagnostics = {}
-  for line in stderr:gmatch('[^\r\n]+') do
-    local lnum, col, msg = line:match('Error at .+ %(line (%d+), column (%d+)%): (.+)$')
+  local lines = vim.split(output, '\n')
+  local i = 1
+  while i <= #lines do
+    local line = lines[i]
+    local lnum, col, msg = line:match('%(line (%d+), column (%d+)%):%s*(.*)$')
     if lnum then
-      table.insert(diagnostics, {
-        lnum = tonumber(lnum) - 1,
-        col = tonumber(col) - 1,
-        message = msg,
-        severity = vim.diagnostic.severity.ERROR,
-      })
-    else
-      local ylnum, ycol, ymsg =
-        line:match('YAML parse exception at line (%d+), column (%d+)[,:]%s*(.+)$')
-      if ylnum then
-        table.insert(diagnostics, {
-          lnum = tonumber(ylnum) - 1,
-          col = tonumber(ycol) - 1,
-          message = ymsg,
-          severity = vim.diagnostic.severity.ERROR,
-        })
-      else
-        local errmsg = line:match('^pandoc: (.+)$')
-        if errmsg and not errmsg:match('^Error at') then
-          table.insert(diagnostics, {
-            lnum = 0,
-            col = 0,
-            message = errmsg,
-            severity = vim.diagnostic.severity.ERROR,
-          })
+      if msg == '' then
+        for j = i + 1, math.min(i + 2, #lines) do
+          local next_line = lines[j]:match('^%s*(.+)$')
+          if next_line and not next_line:match('^YAML parse exception') then
+            msg = next_line
+            break
+          end
         end
       end
+      if msg ~= '' then
+        table.insert(diagnostics, {
+          lnum = tonumber(lnum) - 1,
+          col = tonumber(col) - 1,
+          message = msg,
+          severity = vim.diagnostic.severity.ERROR,
+        })
+      end
+    else
+      local errmsg = line:match('^pandoc: (.+)$')
+      if errmsg then
+        table.insert(diagnostics, {
+          lnum = 0,
+          col = 0,
+          message = errmsg,
+          severity = vim.diagnostic.severity.ERROR,
+        })
+      end
     end
+    i = i + 1
   end
   return diagnostics
 end
@@ -100,8 +103,8 @@ M.typst = {
   output = function(ctx)
     return (ctx.file:gsub('%.typ$', '.pdf'))
   end,
-  error_parser = function(stderr)
-    return parse_typst(stderr)
+  error_parser = function(output)
+    return parse_typst(output)
   end,
   open = true,
 }
@@ -121,8 +124,8 @@ M.latex = {
   output = function(ctx)
     return (ctx.file:gsub('%.tex$', '.pdf'))
   end,
-  error_parser = function(stderr)
-    return parse_latexmk(stderr)
+  error_parser = function(output)
+    return parse_latexmk(output)
   end,
   clean = function(ctx)
     return { 'latexmk', '-c', ctx.file }
@@ -141,8 +144,8 @@ M.markdown = {
   output = function(ctx)
     return (ctx.file:gsub('%.md$', '.html'))
   end,
-  error_parser = function(stderr)
-    return parse_pandoc(stderr)
+  error_parser = function(output)
+    return parse_pandoc(output)
   end,
   clean = function(ctx)
     return { 'rm', '-f', (ctx.file:gsub('%.md$', '.html')) }
@@ -171,8 +174,8 @@ M.github = {
   output = function(ctx)
     return (ctx.file:gsub('%.md$', '.html'))
   end,
-  error_parser = function(stderr)
-    return parse_pandoc(stderr)
+  error_parser = function(output)
+    return parse_pandoc(output)
   end,
   clean = function(ctx)
     return { 'rm', '-f', (ctx.file:gsub('%.md$', '.html')) }