Account for hybrid cache entry overhead

[mjc]

Summary

• charge hybrid cache entries for key, metadata, and encoded payload framing
• keep capacity accounting aligned with retained memory rather than payload bytes alone

Branch-added tests

• A focused weigher fixture compares the exact encoded entry size with the key and in-memory metadata overhead and verifies the hybrid weight includes all components.
• Existing cache serialization and capacity tests remain green.

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[Copilot]

🟡 Changes recommended

The new weigher/test currently uses String::len() instead of capacity(), which can undercount retained heap memory and misalign eviction/capacity accounting.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

This PR updates hybrid cache capacity accounting so entry weights reflect not only article payload bytes, but also key storage, in-memory metadata, and serialized framing overhead—keeping eviction behavior better aligned with actual retained footprint.

Changes:

• Introduce a shared hybrid_entry_weight used by both production and memory-only hybrid cache builders.
• Add DiskCachedArticle::encoded_len() to measure serialized value size including metadata and payload framing.
• Add a focused unit test to validate the new weight calculation behavior.

File summaries

File	Description
src/cache/hybrid.rs	Switches foyer weighter from payload-only to a composite entry weight and adds a unit test for the new accounting.
src/cache/hybrid_codec.rs	Adds encoded_len() and a fixed-size constant to represent serialized entry overhead for weight calculations.

Review details

Suppressed comments (1)

src/cache/hybrid.rs:720

• This test hard-codes short_key.len() in the expected weight. If the weighter is meant to track retained memory, it should use capacity() (matching the production code) so the assertion remains correct for strings with reserved capacity.

            size_of::<String>()
                + short_key.len()
                + size_of::<DiskCachedArticle>()
                + missing.encoded_len()

• Files reviewed: 2/2 changed files
• Comments generated: 1
• Review effort level: Lite

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