fix(builtins): resolve 10 eval-surfaced interpreter bugs

[chaliy]

Summary

• Fix 7 bugs across tail, tr, grep, sed, awk, and the interpreter, unignoring all 10 eval-surfaced test cases from test: add skipped tests for eval-surfaced interpreter bugs #214
• Add BRE (Basic Regular Expression) support to grep and sed — literal ( ) are now correctly distinguished from group metacharacters \( \)
• Implement VFS script execution by absolute path after chmod +x

Changes

Bug	Fix	Files
tail -n +N returns wrong lines	Parse +N prefix, add take_from_line()	headtail.rs
tr '[:lower:]' '[:upper:]' garbled	Implement POSIX character class expansion	cuttr.rs
grep 'feat(' crashes (unclosed group)	Add bre_to_ere() conversion in default mode	grep.rs
sed BRE capture groups fail with literal ()	Add bre_to_ere() conversion for BRE patterns	sed.rs
awk match($0, /re/, arr) produces no output	Support 3rd-arg capture array (gawk extension)	awk.rs
/tmp/script.sh after chmod +x → command not found	Check VFS execute permission, parse and run script	mod.rs
awk '{total += $2 * $3}' wrong result	Already worked — test was incorrectly skipped	—
while read in pipe empty vars	Already worked — test was incorrectly skipped	—
sed -E ERE capture groups	Already worked — test was incorrectly skipped	—

Skipped test count: 87 → 77 (bash comparison: 488 → 492 matching real bash)

Test plan

• cargo fmt --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test --all-features — all pass
• All 10 previously-skipped spec tests now pass
• Bash comparison tests: 492/492 match real bash (100%)