refactor(parser): extract helpers from three oversize parsers

[mpecan]

Summary

Extracts helpers from three parser functions that each carried #[allow(clippy::too_many_lines)]:

Function	Before	After	File
parse_simple_command	88 lines	55 lines	src/parser/simple_command.rs
parse_cond_primary	79 lines	13 lines	src/parser/conditionals.rs
parse_coproc	114 lines	27 lines	src/parser/functions.rs

All three #[allow] attributes dropped.

Shared

• src/parser/helpers.rs: new pub(super) const fn is_redirect_op_kind(TokenType) -> bool collapses the 12-variant redirect-operator matches! used in 3 places (simple_command.rs, functions.rs, and redirects.rs::is_redirect_operator).

simple_command.rs

• try_parse_fd_redirect(&mut self, tok) -> Result<Option<Node>> — encapsulates the adjacent-fd / varfd redirect lookahead.
• build_word_node(Token) -> Node — span-preserving Word node builder, documented distinct from helpers::word_node_from_token (which uses Node::empty).

conditionals.rs

• try_parse_cond_negation, try_parse_cond_group, parse_cond_operand methods extract the !, (...), and operand-dispatch branches. Dispatcher extracts TokenType (Copy) to avoid borrow-vs-mut conflicts with the peeked &Token.

functions.rs

• coproc_starts_command(TokenType) -> bool (free const fn) unifies the two starts_command() && !matches!(Coproc|Time|Bang) checks.
• build_coproc_with_command, parse_coproc_redirect_only, parse_coproc_word_loop, build_coproc_synthetic_command methods split the 4 dispatch paths (A/B/C/D) of parse_coproc.

Test plan

• cargo fmt
• cargo clippy --all-targets -- -D warnings — no warnings
• cargo test — 252 passed
• Oracle suite — 12 oracle_* tests pass (cargo test --test integration oracle_)
• AST is byte-identical (constitutional: compatibility is correctness)

Stack

Part of the v0.2.0 refactoring cycle (#61). This is PR 8 of 10.

Closes #58

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