Build natively for arm64 and repair InputMethodKit candidate commits

[akiltipu]

The legacy project does not build natively for arm64 with the selected toolchain. After enabling arm64, Bengali suggestions appeared on macOS 27 but committing with Space or clicking a candidate inserted nothing. LLDB captured -[__NSSingleObjectArrayI insertText:replacementRange:]: unrecognized selector: the cached client was stale, while InputMethodKit's current client was valid.

This keeps the existing Objective-C application and makes a small product change:

• Select arm64 in Debug and Release.
• Use [self client] for candidate/delimiter insertion.
• When the candidate panel has no selection, commit the remembered candidate if valid, otherwise the first; preserve explicit selections and avoid empty-list insertion.

Parser rules, dictionaries, UI resources, dependencies and deployment settings are unchanged. The current SDK supplies an inherited deployment target of 27.0; no lower-OS runtime support is claimed. This branch targets arm64 only.

Validation

• Fresh Debug and Release builds: PASS with native arm64 macOS 27.0.1 (26A434), Xcode 27.0 (27A266a), SDK 27.0 (26A425), Apple clang 21.0.0.
• ./scripts/check.sh: eight controller cases, 15 exact UTF-8 snapshots captured from unchanged upstream parser/rules, packaged resources, SQLite integrity, ICU/dictionary lookup and autocorrect checks PASS. The controller harness fails against unchanged upstream as expected.
• One bundled Mach-O is arm64; all nine direct system dependencies pass native loaded-header inspection. No non-system library paths or LC_RPATH.
• The user reports the requested TextEdit, Chrome and VS Code typing matrix works with the repaired Debug installation. This is grouped manual confirmation, not automated per-keystroke evidence.
• Both full builds retain 46 existing warnings. No signing bypass was needed; these are local ad-hoc builds, including get-task-allow, not distribution artifacts.

The PR adds isolated tests and a nonprivileged ARM64 GitHub Actions workflow. Remote CI is pending its first run. Fork-specific branding/release documents are excluded.

Review boundaries

This is an Objective-C alternative to the broader changes currently in #2 and #3, whose heads include the Swift migration. No commits were copied from those forks.

Release runtime, preference/learning persistence, clean-account registration, full rollback and notarized/downloaded-artifact tests remain NOT TESTED. A public binary is not included. Credits states MPL 1.1 for the application, but clarification is still needed for the exact scope covering dictionaries/rules and NSString+Levenshtein. A further inspection of the original RTF hyperlink confirms CC BY 3.0 for the Dortmund icons. Existing notices are preserved; this PR does not assign new terms to inherited material.

Fork-specific prerelease preparation is separate: akiltipu#1. That branch replaces the historical distance routine; it is not part of this smaller compatibility PR.