fix(macos): restore share blocks and handle Mac-compressed ZIPs

[theincrediblewwz]

Problem and behavior

Mac share import could lose Windows block boundaries and fail on folders compressed by macOS when UTF-8 filenames lacked the ZIP language flag. Import now restores top-level exported block IDs, source types, order and separate Markdown files; Mac ZIP export retains that metadata. Legacy unmarked Markdown remains one block, and merged continuation groups without internal offsets remain intact with a notice.

The native picker now opens asynchronously after Notebooks dismissal, attaches to the initiating workspace through a stable window anchor, and filters folders/ZIP/Markdown without mixed inferred UTIs. Main-window ZIP/folder drops also use the share importer. ZIP filename decoding honors Unicode path metadata, accepts strictly valid UTF-8 without bit 11, and retains legacy CP437 fallback plus all existing path/size/CRC checks.

Validation

• Core: 425 passed, 2 skipped. One cancellation test timed out on an earlier concurrent run; its 31-test file and the subsequent complete suite passed without changing the test.
• Native contract 99 and UI contract 205 checks passed.
• Real native preview regression passed, including window-anchor lifecycle, Windows block IDs, image rendering and ZIP reimport.
• Local native release build and package contract passed.
• In an isolated GUI, Windows-format and actual macOS ditto ZIP fixtures both restore three blocks, the original image and rendered math. ZIPs were selected with the keyboard.
• Public source gate passed. No Windows or Android application sources changed.

User acceptance and release

On 2026-09-08 the user confirmed foreground file selection and import now work, and requested the ZIP plus GitHub release update. Automated keyboard-based fixture checks and this human mouse acceptance are recorded separately. README and release documentation now identify macOS 0.3.7. Publish through the normal Mac release workflow after merging; Windows/Android release assets remain unchanged.