fix(interpreter): correct left-to-right redirect ordering for fd dup + file combos

[chaliy]

Summary

Closes #853

• Fixes result=$(cmd 2>&1 >file) which incorrectly captured empty string instead of stderr
• Adds fd-table model for redirect processing when DupOutput and file redirects are mixed
• Corrects dev_null tests that asserted old (incorrect) behavior for >&2 2>/dev/null patterns

What

Bash processes redirects left-to-right, building an fd table where each dup copies the current target of the source fd. For 2>&1 >file:

1. 2>&1 — fd2 = copy(fd1) = capture pipe
2. >file — fd1 = file

So stderr goes to the capture pipe (result) and stdout goes to the file.

The old implementation mutated stdout/stderr strings sequentially — 2>&1 merged stderr into stdout, then >file wrote the merged string to the file, losing the fd identity.

How

• When redirects contain both DupOutput and file redirects, use a new apply_redirections_fd_table method that builds an abstract fd table first, then routes original stdout/stderr based on the final mapping
• Fast path preserved: when no mixed dup+file redirects exist, the original single-pass logic runs unchanged
• Updated two dev_null tests that tested the old buggy behavior (echo error >&2 2>/dev/null should output to stderr in real bash, not suppress it)

Tests

• redirect_2_to_1_then_file_in_cmdsub — core reproduction from issue
• redirect_2_to_1_then_file_outside_cmdsub — same pattern without command substitution
• redirect_file_then_2_to_1 — reverse order (>file 2>&1 sends both to file)
• test_dev_null_stderr_redirect / test_dev_null_append_stderr — corrected to test proper fd semantics
• Full test suite passes (2500+ tests, 0 failures)