Add zero-copy CBOR views

[ky438]

• add source-level exact span reads and use them for definite-length CBOR text/byte strings
• add raw CBOR view utilities for item spans, skipping, etc
• add coverage for zero-copy cursor views, stream straddling, raw item spans, collation, and invalid view input

built with gpt5.5-xhigh

[ky438]

Follow-up:

• add bulk append support for bytes_sink and use it for CBOR text/byte payload writes
• expose cbor::view::map_entry_view and cbor::view::map_entries() for zero-copy map traversal
• make cbor::view::compare() compare indefinite/chunked text and byte strings semantically
• add tests for map entry spans and semantic chunked string comparison

[ky438]

Follow-up fix for the CIFuzz failure on fuzz_cbor:

• bound stream_source::read_span() growth so malformed huge string lengths cannot trigger an unbounded allocation
• fall back to the existing copy path when a safe span is unavailable
• add a regression test for the fuzzed test case
• avoid exact reserve before bytes_sink insert-based appends
• share CBOR text/byte length header encoding through one helper

[danielaparker]

Thanks! This contribution is very much appreciated.