Add remaining flashcards and reference files (qn_01, segment_tree, ds, learning)

.learning/cpp-pointers-and-references/mastery.md

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+# C++ Pointers and References - Mastery Checklist
+
+## Core Concepts
+
+Track your understanding of key concepts:
+
+- [ ] Declaration syntax: & and * placement
+- [ ] Pointers vs references: mental model
+- [ ] Pointer arithmetic
+- [ ] const correctness with pointers and references
+- [ ] References as function parameters
+- [ ] Pointer-to-pointer
+- [ ] Null pointers and nullptr
+- [ ] Dangling pointers and references
+- [ ] Smart pointers (unique_ptr, shared_ptr, weak_ptr)
+- [ ] Lvalue references vs rvalue references
+- [ ] Move semantics
+- [ ] Perfect forwarding
+- [ ] Reference collapsing rules
+- [ ] Common interview traps