bluejay-parser: performance optimizations

[swalkinshaw]

~16% faster schema parsing, ~12% faster executable parsing

Key optimizations:

• Rewrite block string parser: direct string processing instead of sub-lexer + Vec<Vec> (~10%)
• Compact Span: u32 start+len (8 bytes) instead of Range (16 bytes), add Copy (~3%)
• Field: consume-then-check for alias instead of peek(1) (~2%)
• Optimize next_if_* methods: peek+consume in single buffer operation (~1%)
• Lazy depth_limiter.bump(): only bump when optional elements exist
• Add Copy to DepthLimiter + preallocate Vec capacity in DefinitionDocument

Also adds benchmarks for ExecutableDocument parsing with a large fixture, and updates downstream crates to use Copy semantics on Span (clone → deref).

Note: this is a manually curated (with the help of Claude) and cleaned up version of an /autoresearch run