fix: real timestamps on claude preamble steps; validate asserts schema formats

[od0]

What

Fixes #119.

Claude Code writes several headerless session-log lines with no timestamp field (last-prompt, ai-title, custom-title; a headless claude -p run produces them every time). toolpath-claude stamped the derived claude-preamble-N steps with "timestamp": "", which the schema forbids ($defs/timestamp is format: date-time) — but path p validate accepted anyway, because JSON Schema draft 2020-12 treats format as annotation-only unless the validator opts in. This PR fixes both ends.

• toolpath-claude (0.12.1): a step derived from a timestamp-less preamble line resolves its timestamp from the nearest real source — the entry the line references (leafUuid, which real last-prompt lines carry, or messageId on file-history-snapshot), else the previous preamble line's resolved time, else the session's first timestamped entry. Only the derived step's identity changes; the source line still rides verbatim in the step's structural change (extra["raw"]), so projecting back to Claude JSONL is unaffected — a test proves the raw lines gain no timestamp on the way out.
• path-cli (0.15.0): p validate opts into format assertion, so format: date-time on step timestamps and format: uri on meta.kind / path-ref $ref / signature href are enforced instead of ignored. On a pre-fix import this produces exactly the failure from Claude imports emit empty timestamps on preamble steps, and validate doesn't catch them #119 — a oneOf violation at /paths/0. Documents cached by older toolpath-claude releases may now fail validation on their empty preamble timestamps; re-importing the session fixes them.

Tests

• provider: resolution priority across all four branches (own timestamp > referenced entry > previous preamble line > first entry), plus a projection test proving the timestamp fill doesn't leak into re-emitted JSONL.
• derive: a fixture session with a last-prompt entry — every derived step parses as RFC 3339, and the last-prompt step carries the leafUuid-referenced entry's time.
• real fixture: the captured session (3 last-prompt + 4 ai-title lines) derives with all-valid timestamps.
• schema: an empty step timestamp is rejected, surfacing at /paths/0.
• integration: end-to-end p import claude → p validate on a preamble-bearing fixture — verified red without the provider fix, green with it.

Verification

A fresh headless-session import validates. A real ~300-preamble-line session went from ~300 empty timestamps to zero and validates; a pre-fix import of the same session now fails with the oneOf-at-/paths/0 signature from the issue. All 12 example documents and a live Codex import still validate under format assertion. cargo test --workspace (57 targets), cargo clippy --workspace -- -D warnings, the out-of-workspace toolpath-cli shim build, and the site build (11 pages) are all green.

Scope

• One logic change in toolpath-claude (conversation_to_view), one in path-cli (schema.rs opts into format assertion), plus tests.
• toolpath-claude 0.12.0 → 0.12.1 (patch), path-cli 0.14.0 → 0.15.0 (validate behavior change; pre-1.0 minor), toolpath-cli shim tracking 0.15.0 — with the workspace / crates.json / CHANGELOG / CLAUDE.md updates.
• Known adjacent gap, deliberately not in this PR: toolpath-cursor builds turn timestamps with created_at.unwrap_or_default() and could emit the same class of empty timestamp if a bubble ever lacks createdAt.

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