fix(builtins): clear variable on read at EOF with no remaining data

[chaliy]

Summary

• Fix read at EOF retaining the last value instead of clearing the variable
• When read reaches EOF with no data, now clears target variables to empty before returning 1
• Prevents the common while read line || [[ -n "$line" ]] pattern from infinite looping

What

The read builtin returned ExecResult::err("", 1) on EOF without any SetVariable side effects, so the variable kept its previous value. Now the EOF path parses flag arguments first, then emits SetVariable side effects with empty values for all target variables.

Test plan

• New spec test: read_eof_clears_variable — verifies printf "one\ntwo" loop terminates
• New spec test: read_eof_partial_line — verifies partial line without trailing newline is captured
• All existing read-builtin spec tests pass
• Manual smoke test confirms no infinite loop

Closes #953