fix(semantic): preserve Lua return rows

[lewis6991]

Problem

Lua return values were collapsed into one LuaType too early. That made row
shape depend on the consumer and blurred cases that Lua keeps distinct:

• zero returned values vs one returned nil
• exhausted fixed return slots vs an explicit returned value
• empty or unbounded generic result rows such as R...

For example:

---@return
local function none() end

---@return nil
local function one_nil() end

arity(none())    -- zero arguments
arity(one_nil()) -- one nil argument

Solution

• Store function and signature results as adjusted return rows.
• Collapse a row only when a caller asks for a single expression value.
• Centralize row merging, overload slot lookup, and return-count min/max logic.
• Apply Lua expression-list adjustment from the shared inference path:
  • only the final expression may expand
  • non-final multi-returns use slot 0
  • exhausted fixed slots become nil
• Update diagnostics to count adjusted rows instead of guessing from collapsed
  variadic types.

Validation

• cargo test -p emmylua_code_analysis

[gemini-code-assist]

Code Review

This pull request implements a major refactor of the type inference engine to accurately model Lua's multi-return semantics by replacing single return types with "return rows" (Vec). A new return_row module provides logic for merging rows, adjusting result slots, and handling variadic tails, which is now integrated across closure analysis, generic instantiation, and assignment logic. These changes ensure that tail calls, nil padding in assignments, and arity preservation in higher-order functions are correctly handled. Additionally, diagnostic checkers for parameter and return counts were updated to utilize this new row-based logic, supported by an extensive suite of new test cases. I have no feedback to provide.