feat: add optional jiff 0.2 shape support

[tisonkun]

Summary

• add a no_std-compatible jiff02 feature backed by Jiff 0.2 without enabling its default features
• expose string serialization and deserialization shapes for all eight Jiff types with direct Serde implementations
• exercise the feature in the test task and on the no_std CI target
• document the exact integration boundary

The implementation deliberately does not describe Jiff's alternate serde adapters: those remain custom-function boundaries unless a consumer supplies an explicit serde_shape hook.

Review order

1. c4c99b7 feat: add optional Jiff 0.2 shape support
   • Why: downstream crates cannot implement serde-shape traits for foreign Jiff types, and Jiff's direct Serde implementations have a stable string boundary.
2. 29e0b4d docs: describe the Jiff integration
   • Why: users need to know which Jiff version and concrete types the optional feature covers.

Validation

• cargo x lint
• cargo x build --locked
• cargo x test
• cargo x package --locked
• cargo +1.85.0 x test
• cargo +1.85.0 check -p serde-shape --lib --no-default-features --features jiff02 --target thumbv7em-none-eabihf