fix: use shared auto-compact threshold constants

[DeepamJha]

Summary

• Use COMPRESSION_CONSTANTS as the single source of truth for auto-compact threshold clamping.
• Update boundary tests to derive expected values from the shared constants.
• Prevent threshold ranges from drifting between implementations.

Closes #1137

Validation

• Focused AVA suite: 41 passing
• Lint: passed
• Format: passed
• pnpm test:all: blocked by intermittent Corepack/pnpm signing-key verification.

[github-actions]

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