feat: unified reload field for live-preview (SSE + long-running watch) (#19)

* feat(reload): add SSE live-reload server module

Problem: HTML output from pandoc has no live-reload; the browser must
be refreshed manually after each compile.

Solution: add lua/preview/reload.lua — a minimal SSE-only TCP server.
start() binds 127.0.0.1:5554 and keeps EventSource connections alive;
broadcast() pushes a reload event to all clients; inject() appends an
EventSource script before </body> (or at EOF) on every compile so
pandoc overwrites do not lose the tag.

* refactor(presets): add reload field, remove synctex field

Problem: the synctex field only handled PDF forward search and left
HTML live-preview and typst watch mode unsupported.

Solution: add reload = function(ctx) returning { 'typst', 'watch',
ctx.file } to typst (long-running watch mode), reload = true to
markdown and github (SSE push after each pandoc compile), and remove
synctex = true from latex (the -synctex=1 arg in latex.args remains
for .synctex.gz generation).

* refactor(init): replace synctex field and validation with reload

Problem: ProviderConfig still declared synctex and validated it, but
the field is being dropped in favour of the general-purpose reload.

Solution: replace the synctex annotation and vim.validate call with the
reload field, accepting boolean | string[] | function.

* feat(compiler): support long-running watch processes and SSE reload

Problem: compile() only supports one-shot invocations, requiring a
BufWritePost autocmd for watch mode and leaving HTML without live-
reload.

Solution: resolve_reload_cmd() maps provider.reload (function or table)
to a command; when present, compile() spawns it as a long-running
process instead of building a one-shot cmd from provider.cmd + args.
toggle() detects long-running providers and toggles the process
directly instead of registering a BufWritePost autocmd. When
reload = true and output is .html, the SSE server is invoked after
each successful compile. status() reports is_reload processes as
watching, not compiling. stop_all() also stops the SSE server.

* fix(compiler): format is_longrunning and annotate is_reload field

Problem: stylua required is_longrunning to be on one line; lua-ls
warned about undefined field is_reload on preview.Process.

Solution: inline the boolean expression and add is_reload? to the
preview.Process annotation.

* refactor: rename compile/toggle commands to build/watch

Problem: `compile` and `toggle` are accurate but unintuitive — `compile`
sounds academic and `toggle` says nothing about what it toggles.

Solution: rename the public API and `:Preview` subcommands to `build`
(one-shot) and `watch` (live preview). Internal compiler functions are
unchanged. No aliases for old names — clean break.

lua/preview/presets.lua

@@ -107,6 +107,9 @@ M.typst = {
     return parse_typst(output)
   end,
   open = true,
+  reload = function(ctx)
+    return { 'typst', 'watch', ctx.file }
+  end,
 }
 
 ---@type preview.ProviderConfig
@@ -117,6 +120,7 @@ M.latex = {
     return {
       '-pdf',
       '-interaction=nonstopmode',
+      '-synctex=1',
       '-pdflatex=pdflatex -file-line-error -interaction=nonstopmode %O %S',
       ctx.file,
     }
@@ -150,6 +154,7 @@ M.markdown = {
     return { 'rm', '-f', (ctx.file:gsub('%.md$', '.html')) }
   end,
   open = true,
+  reload = true,
 }
 
 ---@type preview.ProviderConfig
@@ -179,6 +184,7 @@ M.github = {
     return { 'rm', '-f', (ctx.file:gsub('%.md$', '.html')) }
   end,
   open = true,
+  reload = true,
 }
 
 return M