feat: add parse_lua_surface (S2 Phase 1)

Extracts M.<name> = function(...) patterns from Lua source; classifies
underscore-prefix as private. Pure regex-based (Lua patterns); 95%
coverage for idiomatic Sporel-Lib code per spec §4.3.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

init.lua

@@ -2,4 +2,20 @@
 -- See: meta/docs/superpowers/specs/2026-05-16-api-doc-convention-design.md
 local M = {}
 
+-- Extracts public/private surface from Lua source code.
+-- Convention: `M.<name> = function(...)` is public; `M._<name> = ...` is private.
+-- Returns: { public = ["foo","bar",...], private = ["_baz",...] }
+function M.parse_lua_surface(source_string)
+    local public  = {}
+    local private = {}
+    for name in string.gmatch(source_string, "M%.([_%w]+)%s*=%s*function") do
+        if string.sub(name, 1, 1) == "_" then
+            table.insert(private, name)
+        else
+            table.insert(public, name)
+        end
+    end
+    return { public = public, private = private }
+end
+
 return M