fix: validate marks immediately after creation in visual selection

[funkybooboo]

Fixes crashes when marks are used before validation during visual selection operations.

Problem

The current code creates marks and immediately uses them for logging and RequestStatus before validating they were created successfully. When the buffer is modified between range calculation and mark creation, the marks fail to create but are still used, causing crashes.

Current flow:

1. Create marks (may fail silently)
2. Use marks for logging - crashes if invalid
3. Pass marks to RequestStatus - crashes if invalid
4. Start status updates - crashes if invalid
5. Much later, in async callback, validate marks (too late)

Solution

Validate marks immediately after creation, before using them. This fails fast with a clear error message instead of crashing later with cryptic errors.

New flow:

1. Create marks
2. Validate marks immediately
3. If invalid, log fatal error and return early
4. If valid, proceed safely knowing marks are good

Benefits

• Fails fast with clear error message
• No defensive nil checks needed throughout codebase
• Single validation point instead of scattered checks
• Prevents crashes in logging, RequestStatus, and status updates
• Better user experience with actionable error message

Testing

• All existing tests pass (68/68)
• Linting passes
• Formatting passes
• Follows existing error handling patterns in the codebase

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