Continue V3.3 localization and release preparation

[sanskarIN]

Goal

Continue issue #102 from the merged searchable-language-picker foundation and prepare the next V3.3 release line without prematurely changing the package version.

Implemented foundation in this branch

• add Flutter source-generated UI localization in the application source tree
• keep UI locale independent from spelling/document language
• keep English as the only supported UI locale until a complete reviewed non-English surface is ready
• localize the application title/keyboard-help surface and reusable searchable spelling-language picker
• add focused localization and locale-independence regressions
• document the localization contribution, compatibility, and privacy boundary

Corrective integration work in progress

A post-commit audit found that an earlier bootstrap script masked a failed source replacement because the shell block did not fail fast. The commit labelled as picker integration had only formatting changes, while the editor still rendered the legacy dropdown. That result is not being treated as valid.

A strict corrective pass now:

• wires SpellingLanguageSelector into the real editor flow
• explicitly rejects any remaining legacy DropdownButton<String> language selector
• synchronizes the exact executable-build tracked-file inventory with the new localization files
• updates runtime-dependency documentation
• uses set -euo pipefail
• runs generated localization, formatting, analyzer, focused picker/localization tests, the complete Flutter suite, and benchmark smoke before pushing corrections

GitHub-hosted jobs are currently queued; the branch will remain draft until an actual strict run executes successfully.

Versioning gate

The package remains 3.2.0+25. The coordinated 3.3.0+26 identity bump will happen only after the V3.3 behavior/documentation/accessibility/six-platform gates are green and the selected first non-English UI scope is complete enough to avoid a partially translated release.

Part of #102.