Warn when the installation ID can't be saved; add require_persisted - #7

Problem

identity.get_or_create_installation_id() swallowed write errors and returned an unsaved ID. On a read-only disk, in a sandbox, or without permission to write ~/.keymint, every launch generated a different ID, so the same machine used another activation slot each time, and the app got no signal.

Change (backwards compatible)

• Default unchanged. The function still returns an ID and never raises because of a failed save. It now emits one InstallationIdWarning (a RuntimeWarning subclass) per process naming the storage path.
• Opt-in strict mode. New keyword-only argument require_persisted=False. With True it raises InstallationIdError when the ID can't be saved and read back, or an existing file can't be read. InstallationIdError subclasses OSError (so existing except OSError handlers still catch it) and has path.
• After writing, the value is read back to confirm it persisted.
• Return type unchanged (str).
• InstallationIdError and InstallationIdWarning are exported from keymint.
• Docstring, README ("When the installation ID can't be saved") and CHANGELOG (Unreleased) describe the default, the warning and the strict option, and recommend require_persisted=True for desktop apps that activate devices.

No version bump.

Tests

New tests/test_identity.py:

• saves normally and returns the same ID next time (default and strict);
• an existing readable file is reused;
• unwritable path in default mode returns an ID and warns exactly once;
• strict mode raises InstallationIdError with the path;
• read-only directory via chmod (skipped on Windows or as root);
• strict mode raises when the value doesn't read back.

pytest -q tests: 97 passed. ruff check .: clean.