Fix: allow trailing-dot float literals

[SavioCodes]

Summary

• allow float literals with a trailing dot such as 0.
• keep malformed numeric patterns like 4..0 rejected
• avoid stealing the dot from postfix-style token sequences such as pair.0.age
• add regression coverage in lexer/parser plus a compileable sample source

Why This Matters

• trailing-dot floats are a reasonable surface improvement on their own
• the lexer rule is shaped so it does not paint tuple/postfix parsing into a corner later
• the PR stays small and focused on lexer/syntax instead of reopening tuple access itself

Validation

• cargo fmt --all
• cargo test -p frontend --lib
• cargo test -p frontend --test parser --no-run
• cargo test --test trailing_float --no-run
• cargo test --no-run
• cargo clippy --all-targets --all-features -- -D warnings
• git diff --check

Local Note

• integration test executables are compiled locally with --no-run because this machine blocks some test binaries with Windows App Control (os error 4551)

[SavioCodes]

The lexer accepts