[codex] add optional mock labels

[Old-Ding]

Summary

Closes #43.

This adds optional labels to the generic mock state so missing-expectation failures can identify which mock instance failed when a test uses several mocks at once. Unlabeled mocks retain their existing panic messages.

Changes

• Added Generic::with_label(expected, label) while keeping Generic::new(expected) unchanged.
• Added a shared next_expectation(message) helper that appends the label to the existing missing-expectation message when present.
• Routed every Generic-backed expectation path in ADC, I2C, digital, delay, PWM, and SPI mocks through the shared helper.
• Added regression tests for labeled and unlabeled messages, public type-alias usage, ADC, and both embedded-hal I2C generations.

Validation

Local checks passed:

• cargo test --all-features (145 unit tests and 21 doc tests)
• cargo +1.75.0 test --all-features (145 unit tests and 21 doc tests)
• cargo fmt --all -- --check
• git diff --check

[newAM]

new_with_label should be with_label to match naming idioms (such as Vec::new / Vec::with_capacity) but it otherwise looks good to me.

[Old-Ding]

Addressed on the current head (303db16): the public constructor is now Generic::with_label, and no new_with_label references remain. This follows the naming convention you noted.

Verification on the current head:

• cargo test --all-features: 145 unit tests and 21 doc tests passed.
• git diff --check upstream/main...HEAD: clean.