feat(experiment): add durable result recorder

[Staaaaaaaaar]

Motivation

PR 10 requires a durable, versioned filesystem result format that preserves both successful and interrupted experiments and can be validated without a running ROS graph.

Scope

• add a ROS-independent ResultRecorder and typed metric/event/trajectory inputs
• persist canonical config, metadata, Episode specs, events, trajectories, metrics, and summary artifacts
• use atomic sibling-file replacement with fsync and explicit Episode/Experiment commit markers
• safely encode opaque experiment and Episode identifiers as path components
• add an independent strict v1 result reader and validator
• document Result Schema v1 and its commit semantics

Architecture Changes

• metrics.json complete is the authoritative Episode commit marker
• metadata.json complete is the authoritative Experiment commit marker
• files remain the durable source of truth; ROS EpisodeResult publication remains optional
• full orchestrator lifecycle wiring and multi-Episode execution remain in PR 11

Testing

• make dev-lint
• make dev-check
• 16 packages built
• 204 tests passed, 0 errors, 0 failures, 0 skipped
• covered exact layouts, normal and interrupted reads, path traversal IDs, metric/trajectory invariants, strict JSON numbers, atomic replacement, and injected metrics/summary/metadata write failures

Acceptance Criteria

• stable versioned result schema
• exact machine-readable Experiment and Episode layouts
• interrupted output remains parseable with complete=false
• normal output is independently validated
• atomic replacement preserves the previous artifact on commit failure
• path components cannot escape the configured result root

Known Limitations

• no database, object storage, rosbag, or analysis UI
• trajectory output is buffered and committed during Episode finalization
• the recorder is intentionally not wired into the orchestrator until PR 11

Follow-up

PR 11 will integrate the recorder with the full multi-Episode lifecycle and failure policy.

Roadmap: PR 10 - Result Recorder