Add hints for exercises that were missing them

[Nudity0x]

Summary

The README lists "More hints" as the top item under Completion / could still be improved. Most chapters had no hints at all — only variables (full), functions1, templates5, and security had them.

This adds a hint for every exercise that lacked one: 30 new hint files across functions, if, pointers_references, classes, raii, containers, ownership, and templates. Every exercise in the repo now has a matching hint.

Style

Hints follow the existing repo convention: they nudge toward the concept (name the language feature / API, point at the specific broken line, cite the relevant CWE for resource/lifetime bugs) without giving the copy-paste solution. Modeled on the tone of the existing security/* (CWE pointer) and templates5 (skeletal signature) hints.

Notes

• No exercise source, CMake, or existing hint files are modified — this PR is additive only (30 new .md files).
• Nothing to build/verify: hints are documentation. I did cross-check each hint against the exercise's actual broken line and its test's expected values so the guidance is accurate.

Happy to adjust tone/verbosity if you'd prefer hints that reveal more or less.