Make alias expansion accounting aggregate and byte-aware

[jskoiz]

Summary

• Share alias expansion accounting across all aliases and documents in a YAML stream.
• Charge clone work, scalar bytes, and collection-item work before alias materialization for tree and event-backed Serde paths.
• Add focused scalar-byte and multi-document regressions and update the default boundary coverage.
• Refs [P1] Make alias expansion accounting aggregate and byte-aware #78

Verification

• Local: cargo fmt --all -- --check passed.
• Local: git diff --check passed.
• Cargo tests/checks: not run because this shared checkout has no existing target/; the user-directed disk constraint prohibited creating material new build artifacts.
• Hosted CI/merge state: not observed; this is a draft PR and has not been merged.

Notes/Risks

• max_alias_expansion_nodes keeps its public name, but now caps aggregate clone work, scalar bytes, and collection-item work independently across the complete stream.
• The local checkout remains detached because branch creation was blocked by shared Git metadata permissions; the requested remote branch ref is pushed.