Refactor memory management and string handling in console code

[trufae]

Description

This PR refactors memory management and string handling across multiple console modules to improve code quality, fix potential issues, and optimize performance:

Key Changes

libr/cons/grep.c:

• Fix memory leak in colorcode() by freeing res before reassigning buffer
• Improve JSON formatting to use r_str_append() instead of r_str_newf() for efficiency
• Replace strdup() with r_str_ndup() to handle buffer lengths correctly (fixes potential issues with non-null-terminated buffers)
• Remove unnecessary calloc() and strcpy() in zoom handling
• Optimize grep highlighting logic to only allocate string when needed
• Fix buffer length calculation in r_cons_grep_line() to use actual length instead of strlen() for non-null-terminated data
• Defer out buffer allocation until actually needed

libr/cons/rgb.c:

• Refactor r_cons_rgb_tostring() to use early returns instead of multiple conditional assignments
• Fix color comparison logic (e.g., g == b && g == 0 → g == 0 && b == 0)
• Simplify control flow for better readability

libr/cons/canvas.c:

• Replace RStrBuf with stack-allocated buffer for small attribute strings
• Optimize __expandLine() by using memmove() instead of temporary buffer allocation
• Remove unnecessary strdup() in r_cons_canvas_write()
• Reorder null checks in r_cons_canvas_tostring() for clarity

libr/cons/dietline.c:

• Fix buffer length calculation in __delete_current_char() to use actual length instead of strlen()
• Optimize shift_buffer() to calculate length directly instead of calling strlen()
• Fix unix_word_rubout() length calculation
• Replace r_str_pad() with direct printf formatting in history listing
• Cache option string length in selection_widget_select() to avoid repeated strlen() calls
• Add missing free(prompt) in __update_prompt_color()

libr/cons/hud.c:

• Simplify hud_render_prompt() by using pointer arithmetic instead of string duplication

libr/cons/cons.c:

• Replace strlen() with constant for known string length in r_cons_enable_mouse()
• Remove unnecessary string duplication in r_cons_flush()

Benefits

• Eliminates memory leaks and improves resource management

• Fixes handling of non-null-terminated buffers by using length-aware functions

• Reduces unnecessary allocations and string copies

• Improves code clarity and maintainability

• Better performance through optimized string operations

• Mark this if you consider it ready to merge

• I've added tests (optional)

• I wrote some lines in the book (optional)

https://claude.ai/code/session_01RoQJa7ykF4SPBVStcxtJXX