Add #[cbor(text_keys)] for automatic string-keyed map encoding

[ropuls]

Build on top of the #[s("...")] string index support to add a struct/enum-level #[cbor(text_keys)] attribute that automatically uses field/variant names as string keys, removing the need to annotate every field individually.

Features:

• #[cbor(text_keys)] on structs: all fields auto-keyed by name
• #[cbor(text_keys)] on enums: all variants auto-keyed by name
• #[cbor(key = "...")] for custom wire key names (per field/variant)
• text_keys implies map encoding automatically
• Unit enum variants encode as naked strings ("Reset")
• Non-unit enum variants encode as map(1) {"Name": {fields}}

Also fixes:

• Add missing s attribute to CborLen proc_macro_derive

Validation:

• text_keys on tuple/unit structs: compile error
• text_keys + transparent / index_only: compile error
• #[cbor(key = "...")] without text_keys: compile error
• Duplicate key names: compile error

Tests: 40 host tests + 36 STM32H563ZI (Cortex-M33) no_std tests