Security hardening: body size limit, JSON depth limit, timing-safe auth

- Add MAX_BODY_SIZE (64KB) check before reading body — prevents pre-auth
  OOM on internet-facing deployments
- Add JSON nesting depth limit (64 levels) — prevents C stack overflow
  that bypasses pcall and crashes the process
- Timing-safe token comparison via XOR accumulate — prevents timing
  oracle on Bearer token
- Auth token from LMCP_TOKEN env var (highest priority) — avoids storing
  token in a file readable by the read_file tool
- Silent handling of unknown JSON-RPC notifications (spec compliance)
- Exact path matching on /mcp endpoint (was prefix-based)
- Remove dead json.array() function

Findings from architecture review + security audit.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

json.lua

@@ -89,6 +89,7 @@ end
 -- Decode --
 
 local decode_value
+local MAX_DEPTH = 64
 local ws_chars = { [' '] = true, ['\t'] = true, ['\n'] = true, ['\r'] = true }
 
 local function skip_ws(s, pos)
@@ -143,14 +144,14 @@ local function decode_number(s, pos)
     return n, pos
 end
 
-local function decode_array(s, pos)
+local function decode_array(s, pos, depth)
     pos = pos + 1 -- skip [
     local arr = {}
     pos = skip_ws(s, pos)
     if s:sub(pos, pos) == ']' then return arr, pos + 1 end
     while true do
         local val
-        val, pos = decode_value(s, pos)
+        val, pos = decode_value(s, pos, (depth or 0) + 1)
         arr[#arr + 1] = val
         pos = skip_ws(s, pos)
         local c = s:sub(pos, pos)
@@ -160,7 +161,7 @@ local function decode_array(s, pos)
     end
 end
 
-local function decode_object(s, pos)
+local function decode_object(s, pos, depth)
     pos = pos + 1 -- skip {
     local obj = {}
     pos = skip_ws(s, pos)
@@ -174,7 +175,7 @@ local function decode_object(s, pos)
         if s:sub(pos, pos) ~= ':' then error('expected : at ' .. pos) end
         pos = skip_ws(s, pos + 1)
         local val
-        val, pos = decode_value(s, pos)
+        val, pos = decode_value(s, pos, (depth or 0) + 1)
         obj[key] = val
         pos = skip_ws(s, pos)
         local c = s:sub(pos, pos)
@@ -184,12 +185,14 @@ local function decode_object(s, pos)
     end
 end
 
-decode_value = function(s, pos)
+decode_value = function(s, pos, depth)
+    depth = depth or 0
+    if depth > MAX_DEPTH then error("JSON nesting too deep") end
     pos = skip_ws(s, pos)
     local c = s:sub(pos, pos)
     if c == '"' then return decode_string(s, pos)
-    elseif c == '{' then return decode_object(s, pos)
-    elseif c == '[' then return decode_array(s, pos)
+    elseif c == '{' then return decode_object(s, pos, depth)
+    elseif c == '[' then return decode_array(s, pos, depth)
     elseif c == 't' then
         if s:sub(pos, pos + 3) == 'true' then return true, pos + 4 end
     elseif c == 'f' then
@@ -210,9 +213,5 @@ end
 -- Sentinel for JSON null
 json.null = setmetatable({}, { __tostring = function() return 'null' end })
 
--- Helper: encode a table as a JSON array even if empty
-function json.array(t)
-    return setmetatable(t or {}, { __is_array = true })
-end
 
 return json