Preserve phrase buffer after word conversion

[qqrm]

Summary

Preserve the full last-phrase journal sequence after a last-word conversion, and add regression coverage for the double-Shift then double-Alt path.

Problem

After converting only the last word, the journal ended with a programmatic run. Last-sequence extraction preferred that programmatic tail and therefore double-Alt could target only the last word instead of the whole phrase.

Solution

Add a unified last-sequence extractor that keeps a contiguous physical phrase prefix when it is followed by a programmatic tail from word conversion. When converting such mixed-origin sequences, already-programmatic text is preserved so the fixed word is not toggled back while the physical prefix is converted.

Scope

Includes input journal extraction, last-sequence conversion behavior, and regression tests for phrase buffer preservation. Does not change hotkey bindings or selection probing.

Validation

• cargo +nightly fmt --check
• cargo +nightly clippy --all-targets --all-features -- -D warnings
• cargo +nightly build --features debug-tracing
• cargo +nightly test --locked
• actionlint -color
• zizmor --min-severity medium .

Risks

Manual interactive verification in a live target app was not performed in this session; behavior is covered at journal/conversion level.