feat: add validate_readme_structure (S2 Phase 1)

Validates MUST-Sections for tier=core (H1, Badges-Table, Topology-Block,
API, References) + section-order (API before References). Tier=community
returns empty (recommendation-only per spec section 3.1).

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

init.lua

@@ -98,4 +98,35 @@ function M.generate_topology_block(manifest, engine_calls)
     return table.concat(lines, "\n")
 end
 
+-- Validates README structure against MUST-sections for the given tier.
+-- Tier "core" enforces: H1, Abstract, Badges-Table, Topology-Block, API, References.
+-- Tier "community" enforces nothing (returns empty result).
+-- Returns: { missing_sections = [...], section_order_ok = bool }
+function M.validate_readme_structure(markdown_string, tier)
+    if tier ~= "core" then
+        return { missing_sections = {}, section_order_ok = true }
+    end
+
+    local missing = {}
+    local checks = {
+        { name = "H1",             pattern = "^#%s+%S" },
+        { name = "Badges-Table",   pattern = "\n|%s*Field%s*|" },
+        { name = "Topology-Block", pattern = "<!%-%-%s*topology:start" },
+        { name = "API",            pattern = "\n##%s+API[%s\n]" },
+        { name = "References",     pattern = "\n##%s+References[%s\n]" },
+    }
+    for _, c in ipairs(checks) do
+        if not string.find(markdown_string, c.pattern) then
+            table.insert(missing, c.name)
+        end
+    end
+
+    -- Order check: API must appear before References in source order
+    local api_pos = string.find(markdown_string, "\n##%s+API[%s\n]")
+    local ref_pos = string.find(markdown_string, "\n##%s+References[%s\n]")
+    local order_ok = (api_pos and ref_pos and api_pos < ref_pos) or (not api_pos and not ref_pos)
+
+    return { missing_sections = missing, section_order_ok = order_ok }
+end
+
 return M