chore: [#1411] dedup parser boilerplate in cst

[milkyskies]

closes #1411

Summary

Pure refactor — three concrete duplications collapsed in crates/floe-core/src/cst.rs and cst/exprs.rs. Net -23 lines, no behavior change. All parse-tree-shaped tests still pass byte-for-byte.

What changed

Duplication	Fix
expect and expect_kind were byte-identical	Deleted expect_kind, 3 call sites in items.rs migrated to expect
5 left-associative binary parsers (or, and, equality, additive, multiplicative) all followed the same checkpoint + while + bump + recurse template	Extracted parse_left_assoc(ops, next). Each parser is now a one-liner.
Long at(...) || at(...) || ... chains, including a 13-op chain in dot-shorthand-predicate parsing	Added at_any(&[TokenKind]). Applied at the 13-op chain and the 4-op comparison parser.

parse_pipe_expr and parse_comparison_expr opt out of parse_left_assoc because they need extra rules: pipe handles match-into-pipe (x \|> match { ... }), and comparison has the !preceded_by_newline() guard for JSX disambiguation. Both still use at_any internally.

The 2-token Let \|\| Async chains in items.rs were left as || chains — at_any only earns its keep at 3+ alternatives.

Backstory

Spotted by reviewers on #1409 (brace-form record construction): parse_brace_construct_expr and parse_constructor_args shared a lot of scaffolding. That specific dedup is a follow-up issue, but #1411 covered the three lower-hanging duplications in the same file area.

Test plan

• All 1654 core unit tests pass (parse trees byte-identical pre/post)
• All 114 LSP unit tests pass
• Lib clippy clean (cargo clippy --workspace --lib -- -D warnings)
• No example-app changes needed; no docs needed (internal compiler refactor)

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