The REPL previously executed every Enter as a complete command, which forced users to fit conditionals, blocks, and quoted strings on a single line. This change introduces two complementary continuation mechanisms so a logical command can span multiple physical lines.

Parse-driven continuation: a new ShellInterpreter.IsIncompleteInput helper runs the lexer and parser over the buffered text and treats it as incomplete whenever an error has Kind UnexpectedEnd or UnterminatedString. To make that signal reliable, ParseError gains a ParseErrorKind enum (Generic, UnexpectedEnd, UnterminatedString); the seventeen statement_error_unexpected_end* call sites in StatementParser now tag themselves UnexpectedEnd, the missing close-brace branch tags itself UnexpectedEnd when no further token is available, and Lexer.ReadDoubleQuotedString, ReadSingleQuotedString, and ReadInterpolatedString record an UnterminatedString ParseError on EOF exit (new Fluent key lexer_error_unterminated_string). The earlier code silently truncated unterminated strings, so this also fixes a quiet correctness gap.

Explicit backslash continuation: bash-style trailing-backslash continuation is recognized in the REPL loop with proper handling of odd/even backslash runs so '\\\\' at end-of-line stays a literal.

Loop state: ShellInterpreter holds a StringBuilder pendingMultiLineBuffer plus a CosmosShellPrompt reference. While accumulating, the prompt renders '...' in grey. Ctrl+C and a cancelled ReadLine both discard the buffer and clear continuation mode. Command history saves multi-line entries with backslash-encoded newlines (\\\\ and \\n) and decodes on load; single-line entries are unchanged so existing history files stay compatible.

Adds MultiLineInputTests covering unclosed brace, unterminated single/double-quoted strings, balanced input, and empty input. Full suite: 927 passed / 0 failed / 74 skipped.

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