fix: honor COMP_CWORD in generated completion

[codeforester]

Summary

Generate completion input only through the cursor, preserving empty/quoted word boundaries. Keep completion scratch values local while leaving Bash's required COMPREPLY global intact.

Issue

Fixes #508

Validation

• bats lib/bash/cli/tests/lib_cli.bats — passed (37 tests).
• git diff --check — passed.
• Full repository hosted validation is pending.

Demo Impact

None.

Docs Impact

None.

API Impact

No public function signature change; generated completion behavior is corrected.

CI Impact

Adds emitted-function tests for beginning, middle, end, pending-value, nested-command, and -- cursor positions.

Security Notes

None.

[codeforester]

Confirmed fixed. Reproduced the exact repro (COMP_WORDS=(probe admin), COMP_CWORD=2, no trailing placeholder) against the new code — COMPREPLY now correctly completes the empty next word instead of re-suggesting "admin". 37/37 tests pass, including a new test covering this exact boundary.