Preserve conversion hotkeys after cursor movement

[qqrm]

Summary

• Preserve the input journal around Left/Right cursor movement so convert-last-sequence works before the current caret, not only at the journal end.
• Add regression coverage for cursor movement, mid-buffer insertion/backspace, UTF-8 boundaries, suffix preservation, and autoconvert triggers before the caret.
• Add a Windows Sandbox interactive E2E harness for isolated desktop validation of the real keyboard hook and SendInput path.

Problem

Cursor navigation cleared the input journal, so convert word/sequence hotkeys stopped working after moving the caret backward or forward.

Solution

The journal now tracks caret distance from the end, detaches/restores suffix runs around that caret, and extracts the last run/sequence before the caret while preserving text after it. Left/Right update the tracked caret position, End returns it to the journal end, and unsafe navigation keys still clear the journal.

Scope

Included: input journal cursor tracking, regression tests, and an isolated Windows Sandbox E2E harness.

Not included: version bump, release packaging, or a completed sandbox run on this host because Windows Sandbox is not installed/enabled here.

Validation

• cargo +nightly fmt --check
• cargo +nightly clippy --all-targets --all-features -- -D warnings
• cargo +nightly build --features debug-tracing
• cargo +nightly test --locked
• cargo +nightly build --release --locked
• actionlint -color
• zizmor --min-severity medium .
• git diff --check
• PowerShell parse check for scripts/e2e/run-sandbox.ps1 and scripts/e2e/sandbox-runner.ps1
• scripts/e2e/run-sandbox.ps1 -NoWait was attempted and correctly reported missing Windows Sandbox on this host.

Risks

The true interactive E2E scenario still needs to run on a Windows host with Windows Sandbox enabled. Home/Up/Down/Page navigation still invalidates the journal because the hook cannot reliably infer arbitrary line/document positions without a focused-control integration.