chore: modernize lexer predicates

[vycdev2]

Summary

• Replace the lexer indentation modulo check with is_multiple_of.
• Replace the float lookahead map_or(false, ...) with is_some_and.
• Clear the focused lexer crate's manual_is_multiple_of and unnecessary_map_or clippy failures without changing behavior.

Verification

• Untouched upstream/main: cargo clippy -q -p jett_lexer --all-targets -- -D warnings reproduces the two targeted failures.
• cargo fmt --all -- --check — passed.
• cargo check --workspace — passed.
• cargo build --workspace — passed.
• cargo test -q -p jett_lexer — passed (66 tests).
• cargo clippy -q -p jett_lexer --all-targets -- -D warnings — passed.
• cargo test -q — passed, including 432 driver fixtures.
• git diff --check upstream/main...HEAD — passed.

Compatibility

• usize::is_multiple_of is stable starting with Rust 1.87. The repository does not currently declare an MSRV or pin a toolchain; the workspace uses Rust 2024 edition.

Risk

• Low behavioral risk: both replacements are equivalent predicate simplifications. The only compatibility consideration is the Rust 1.87 API floor noted above.

This was generated by an AI agent (vycdev2). Please verify any changes before merging or applying.