refactor(config): replace array preset syntax with preset_name = true (#3)

* refactor(config): replace array preset syntax with preset_name = true

Problem: setup() mixed array entries (preset names) and hash entries
(custom providers keyed by filetype), requiring verbose
vim.tbl_deep_extend boilerplate to override presets.

Solution: unify under a single key=value model. Keys are preset names
or filetypes; true registers the preset as-is, a table deep-merges
with the matching preset (or registers a custom provider if no preset
matches), and false is a no-op. Array entries are dropped. Also adds
-f gfm to presets.github args so pandoc parses input as GFM.

* ci: format

* fix(presets): parenthesize gsub output to suppress redundant-return-value

spec/init_spec.lua

@@ -22,7 +22,7 @@ describe('preview', function()
       assert.are.same({}, config.providers)
     end)
 
-    it('accepts full provider config via hash entry', function()
+    it('merges override table with matching preset', function()
       helpers.reset_config({
         typst = {
           cmd = { 'typst', 'compile' },
@@ -33,8 +33,8 @@ describe('preview', function()
       assert.is_not_nil(config.providers.typst)
     end)
 
-    it('resolves array preset names to provider configs', function()
-      helpers.reset_config({ 'typst', 'markdown' })
+    it('resolves preset = true to provider config', function()
+      helpers.reset_config({ typst = true, markdown = true })
       local config = require('preview').get_config()
       local presets = require('preview.presets')
       assert.are.same(presets.typst, config.providers.typst)
@@ -42,14 +42,14 @@ describe('preview', function()
     end)
 
     it('resolves latex preset under tex filetype', function()
-      helpers.reset_config({ 'latex' })
+      helpers.reset_config({ latex = true })
       local config = require('preview').get_config()
       local presets = require('preview.presets')
       assert.are.same(presets.latex, config.providers.tex)
     end)
 
     it('resolves github preset under markdown filetype', function()
-      helpers.reset_config({ 'github' })
+      helpers.reset_config({ github = true })
       local config = require('preview').get_config()
       local presets = require('preview.presets')
       assert.are.same(presets.github, config.providers.markdown)
@@ -59,7 +59,7 @@ describe('preview', function()
   describe('resolve_provider', function()
     before_each(function()
       helpers.reset_config({
-        typst = { cmd = { 'typst', 'compile' } },
+        typst = true,
       })
       preview = require('preview')
     end)

spec/presets_spec.lua

@@ -139,16 +139,31 @@ describe('presets', function()
       local args = presets.github.args(md_ctx)
       assert.is_table(args)
       assert.are.same({
+        '-f',
+        'gfm',
         '/tmp/document.md',
         '-s',
         '--embed-resources',
         '--css',
-        'https://cdn.jsdelivr.net/gh/pixelbrackets/gfm-stylesheet@master/github.css',
+        'https://cdn.jsdelivr.net/gh/pixelbrackets/gfm-stylesheet@master/dist/gfm.css',
         '-o',
         '/tmp/document.html',
       }, args)
     end)
 
+    it('args include -f and gfm flags', function()
+      local args = presets.github.args(md_ctx)
+      local idx = nil
+      for i, v in ipairs(args) do
+        if v == '-f' then
+          idx = i
+          break
+        end
+      end
+      assert.is_not_nil(idx)
+      assert.are.equal('gfm', args[idx + 1])
+    end)
+
     it('returns html output path', function()
       local output = presets.github.output(md_ctx)
       assert.is_string(output)