fix(parser): reconstruct braces in process substitution token loop

[chaliy]

Summary

• Fix group command { ... } piped to another command producing no output inside process substitution < <(...)
• Root cause: LeftBrace and RightBrace tokens were missing from the process substitution parser's token reconstruction loop, silently dropping them
• Added 3 spec tests covering group+pipe, group+tac, group+sort inside process substitution

What

The process substitution parser reconstructs command strings from tokens to re-parse them. The match block handled Pipe, Semicolon, And, Or, etc. but was missing LeftBrace and RightBrace. These fell through to the catch-all _ branch which silently dropped them, turning { echo a; echo b; } | cat into echo a; echo b; | cat.

Test plan

• 3 new spec tests: process_subst_group_pipe, process_subst_group_pipe_tac, process_subst_group_pipe_sort
• All existing procsub tests still pass
• Manual verification: cat < <({ echo "line1"; echo "line2"; } | cat) now outputs correctly
• cargo fmt --check clean
• cargo clippy clean
• Full test suite green

Closes #947