Changed Diagnostic::msg from String to Cow<'static, str> to avoid
allocations for static error messages. Static messages now use
Cow::Borrowed, while dynamic formatted messages use Cow::Owned.
Also fixed peek_token lifetime issue - was returning reference to
local variable, now returns reference from cache.
Acceptance criteria:
- Token enum with all required variants
- Lexer struct with position tracking and diagnostics
- Diagnostic uses Cow<'static, str> for zero-allocation static messages
- All public methods implemented: new, next_token, peek_token, position, take_diagnostics
- All internal helpers implemented
Refs: pdftract-4hn1
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
Bead-Id: pdftract-4hn1
