refactor(storage): decode checkpoints incrementally

[siriusctrl]

Summary

• replace whole-file message-log reads with a shared synchronous checkpoint decoder and a lazy async forward reader
• publish only complete logical checkpoints while retaining a torn physical line and at most one pending checkpoint
• preserve each committed record's exact raw bytes including LF, source offset, and committed byte end
• make load_trajectory() collect decoded records while prepare_append() uses the same decoder without collecting the full history
• allow validation to start from an arbitrary sequence and byte offset so the follow-up tail-first transcript source can reuse the same checkpoint definition

Why

Resume, append recovery, and the upcoming transcript viewer must agree on the exact committed boundary. The previous tokio::fs::read implementation materialized the whole file and embedded checkpoint validation inside one batch loader, which could not support lazy follow or reverse tail discovery without duplicating rules.

This PR changes storage only. It does not add fmtview, inspect CLI behavior, or TUI code.

Validation

• cargo fmt --check
• cargo check --all-targets
• cargo clippy --all-targets -- -D warnings
• cargo test --test message_log --test message_log_recovery (11 passed)
• cargo test --all-targets (272 passed, 0 failed, 0 ignored)
• echo-provider headless smoke on the rebased commit; inspected messages.jsonl, events.jsonl, run.json, final.md, and the empty artifact directory

The decoder tests cover every byte cut across a torn multi-record tail, malformed and inconsistent completed lines, arbitrary tail offsets, exact tool argument/raw-line preservation, follow completion after EOF, and first-checkpoint delivery without scanning a 20,000-record log.

[siriusctrl]

Independent decoder review found one correctness gap in the reusable tail-candidate seam: CheckpointDecoder validated line_end underflow but not physical continuity, so direct callers could publish raw records with gap/overlap offsets. Draft PR #3 fixes this and adds multi-EOF group completion, deterministic chunk-boundary, large-count, and offset tests. Independent gates on the review commit: fmt/check/strict clippy green; cargo test --all-targets 276 passed, 0 failed, 0 ignored. Recommend merging #3 into this branch before #2.