feat(cell): strengthen LTX runtime foundations

[forhappy]

Summary

• separate logical Cell reads from provider attempts, retain failed outcomes, and keep resident reads at zero provider operations
• emit successful and failed capture ledgers at the ManagedDb boundary with physical WAL file/read attribution
• spool each selected compaction body once under scratch admission, then decode locally while preserving exact-root verification
• update canonical scaling and Celld delta documentation

Verification

• cargo test -p crab-ltx --features replica
• cargo test -p crab-ltx --features replica failed_provider_attempt_is_not_counted_as_a_logical_retry -- --nocapture
• cargo test -p crab-cell-runtime --lib
• cargo test -p crab-http-server --lib completed_request_exports_bounded_full_body_metrics -- --nocapture
• cargo check -p crab-ltx --no-default-features
• cargo clippy -p crab-ltx --features replica --all-targets -- -D warnings
• cargo clippy -p crab-cell-runtime --all-targets -- -D warnings
• cargo clippy -p crab-http-server --lib -- -D warnings