Eliminate fixed-size buffer ceilings and fix deep fd/expansion bugs

Root cause: lexer strips quotes from m["my key"] but inserts char(1)
sentinel at quote boundaries, storing key as "my key\x01". Expansion
path for ${m["my key"]} strips quotes via strip_quotes() producing
"my key" (no sentinel). Key mismatch caused lookup to fail.

Fixes STRESS 24.8: arr[0]=$x now stores full 10000-char values instead of truncating at 9888.

Fixes STRESS 22.3: stderr redirect now persists across loop iterations.
Inner commands no longer clobber outer compound command redirects.

execute_simple_command stack: 171KB -> 13KB, enabling ~400 level
recursion depth (up from 33) with default 8MB stack.