fix: throw a readable error for out-of-range block annotations

[BetterAndBetterII]

Summary

Fixes #486. Out-of-range block annotations (// !mark(100)) crashed in applyBlockAnnotation with Cannot read properties of undefined (reading 'range') when no lines matched. Throw a readable error that names the annotation and the available line count instead.

Test plan

• cd packages/codehike && npx vitest run src/code/lines.test.ts (RED TypeError → GREEN message)
• Existing extract-annotations tests still pass

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