fix(parser): allow comments between a bare return and its terminator

[smn]

Problem

A valueless return followed by a comment before the block terminator (elseif / else / end / until) fails to parse:

if method == "GET" then
    -- ...
    return
-- Handle POST requests
elseif method == "POST" then
    -- ...
end

Parse Error at line ..., column 5: Expected expression

Real Lua 5.3 and Luerl both accept this — comments are whitespace, so the return is simply a bare (valueless) return and the branch continues. This is a common idiom (then return -- note \n elseif ...).

Why it matters for us

This VM runs tenant Lua in production (engage journey engine) and in the lua-dev SDK. The bug surfaced via make test-lua-app APP=healthpass, whose core/init.lua has exactly this shape — the whole module failed to load, reported downstream as a misleading attempt to concatenate a nil value.

Root cause

Lua.Parser.parse_return/1 peeked at the raw next token to decide whether the return was valueless. A comment token isn't one of the terminators, so it fell through to parse_expr_list, which then choked on the comment → "Expected expression".

Fix

Peek past comment tokens (skip_comments/1) when detecting the terminator. The comment tokens stay in the stream, so the block parser still collects them as orphaned/trailing comments (no comment metadata is lost).

case peek(skip_comments(rest)) do

Tests

• New regression test in test/lua/parser/statement_test.exs covering return + comment before elseif/else/end — fails before, passes after.
• mix test test/lua/parser/ — 346 passed
• mix test (full suite) — 2638 passed, 7 skipped
• Verified against the real-world file that surfaced this: all 76 healthpass .lua files now parse.

🤖 Generated with Claude Code

[smn]

Filed upstream as well in tv-labs#418