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/init.lua

@@ -9,6 +9,7 @@
 ---@field errors? false|'diagnostic'|'quickfix'
 ---@field clean? string[]|fun(ctx: preview.Context): string[]
 ---@field open? boolean|string[]
+---@field reload? boolean|string[]|fun(ctx: preview.Context): string[]
 
 ---@class preview.Config
 ---@field debug boolean|string
@@ -34,13 +35,14 @@
 ---@field obj table
 ---@field provider string
 ---@field output_file string
+---@field is_reload? boolean
 
 ---@class preview
 ---@field setup fun(opts?: table)
----@field compile fun(bufnr?: integer)
+---@field build fun(bufnr?: integer)
 ---@field stop fun(bufnr?: integer)
 ---@field clean fun(bufnr?: integer)
----@field toggle fun(bufnr?: integer)
+---@field watch fun(bufnr?: integer)
 ---@field open fun(bufnr?: integer)
 ---@field status fun(bufnr?: integer): preview.Status
 ---@field statusline fun(bufnr?: integer): string
@@ -98,6 +100,7 @@ function M.setup(opts)
       return x == nil or x == false or x == 'diagnostic' or x == 'quickfix'
     end, 'false, "diagnostic", or "quickfix"')
     vim.validate(prefix .. '.open', provider.open, { 'boolean', 'table' }, true)
+    vim.validate(prefix .. '.reload', provider.reload, { 'boolean', 'table', 'function' }, true)
   end
 
   config = vim.tbl_deep_extend('force', default_config, {
@@ -141,7 +144,7 @@ function M.build_context(bufnr)
 end
 
 ---@param bufnr? integer
-function M.compile(bufnr)
+function M.build(bufnr)
   bufnr = bufnr or vim.api.nvim_get_current_buf()
   local name = M.resolve_provider(bufnr)
   if not name then
@@ -173,7 +176,7 @@ function M.clean(bufnr)
 end
 
 ---@param bufnr? integer
-function M.toggle(bufnr)
+function M.watch(bufnr)
   bufnr = bufnr or vim.api.nvim_get_current_buf()
   local name = M.resolve_provider(bufnr)
   if not name then