fix(mpack): boundary values for negative integer encoding #37255

Problem:
libmpack encodes boundary values -129 and -32769 with wrong integer
sizes:
- -129 as int8 instead of int16
- -32769 as int16 instead of int32
because the boundary checks compare against the wrong values (e.g., lo
< 0xffffff7f instead of lo < 0xffffff80). This caused data corruption:
-129 would decode as 127.

Solution:
Fix off-by-one errors in the two's complement boundary constants:
0xffffff80 (-128, min int8) and 0xffff8000 (-32768, min int16).

Fixes #37202

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

test/functional/lua/mpack_spec.lua

@@ -28,6 +28,31 @@ describe('lua vim.mpack', function()
     )
   end)
 
+  it('encodes negative integers at type boundaries correctly #37202', function()
+    -- Test boundary values between int8/int16/int32
+    -- int8 range: -128 to -33 (fixint handles -32 to -1)
+    -- int16 range: -32768 to -129
+    -- int32 range: -2147483648 to -32769
+    local result = exec_lua(function()
+      local tests = {
+        { -128, -128 }, -- int8 boundary (minimum int8)
+        { -129, -129 }, -- int16 boundary (one past int8)
+        { -32768, -32768 }, -- int16 boundary (minimum int16)
+        { -32769, -32769 }, -- int32 boundary (one past int16)
+      }
+      local results = {}
+      for _, test in ipairs(tests) do
+        local input, expected = test[1], test[2]
+        local decoded = vim.mpack.decode(vim.mpack.encode(input))
+        table.insert(results, { input = input, decoded = decoded, ok = decoded == expected })
+      end
+      return results
+    end)
+    for _, r in ipairs(result) do
+      eq(true, r.ok, string.format('encode/decode %d returned %d', r.input, r.decoded))
+    end
+  end)
+
   it('encodes dict keys of length 20-31 as fixstr #32784', function()
     -- MessagePack fixstr format: 0xa0 | length (for lengths 0-31)
     -- Before #36737, strings 20-31 bytes were incorrectly encoded as str8 (0xd9, len)