fix: handle N base in bit-packed insertion encoding

[nh13]

Summary

• Handles 'N' and unknown bases in insertion encoding that would cause bit overflow
• Substitutes random base (matching behavior when bases=NULL) instead of silent corruption
• Fixes both short insertions (line 318) and long insertions (line 350)

Fixes #108

Test plan

• Unit tests pass (make test-unit)
• Integration tests pass (make test-integration)

🤖 Generated with Claude Code

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📥 Commits

Reviewing files that changed from the base of the PR and between 26e3aa9 and 672ef85.

📒 Files selected for processing (1)

• src/mut.c

✨ Finishing touches

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• Commit unit tests in branch fix/bit-shift-n-base

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