refactor(lexer): drop Result<Token> wrapper from operator readers

[mpecan]

Closes #51 · Part of #61 (v0.2.0 Refactoring Cycle, PR 1 of 10)

Summary

Pure signature refactor of src/lexer/operators.rs:

• Change all five operator-reader functions (read_pipe_operator, read_ampersand_operator, read_semicolon_operator, read_less_operator, read_greater_operator) from Result<Token> to Token. Drop the five #[allow(clippy::unnecessary_wraps)] attributes — the fallibility was fiction.
• Introduce a private Lexer::command_token helper that centralises the command_start = true; reserved_words_ok = true transition by delegating to the existing set_command_start() primitive. Nine list-separator branches now route through it; file-redirect branches keep plain Token::new.
• Dispatcher in lexer/mod.rs wraps the now-infallible calls in Ok(...), matching the style already used for (, ), and \n.

Net diff: +98 / −61 across three files; −6 #[allow(clippy::*)] attributes.

Test plan

• cargo fmt && cargo clippy --all-targets -- -D warnings — clean.
• cargo test — 243 passed, 0 failed (was 241; +2 new guard tests).
• Downstream smoke: cargo check --all-targets and cargo test against ../rippy (1332 tests) with rable patched to this branch — all green.
• Downstream smoke: cargo check --all-targets -p tokf against ../tokf with rable patched to this branch — clean.
• grep -rn "unnecessary_wraps" src/lexer/ — zero matches.

New guard tests

Two tests added to src/lexer/tests.rs pin the invariant this refactor centralises:

• list_separators_re_arm_reserved_words — |&, &&, ||, ;;, ;&, ;;& all re-arm reserved-word recognition after foo= clears it.
• file_redirects_do_not_re_arm_reserved_words — >, >>, &>, <, <<< leave the flag clear, so a following for stays a plain Word.

Raised by Agent 4 of the multi-agent review: the newly centralised contract was only covered indirectly via the Parable corpus. These lock it in at the lexer level.

Stack

Fresh from main; no stack.