fix(dashboards): replay keyed create after concurrent title activation

[VibeSWE]

A concurrent keyed create can read an absent dashboard, then lose the title claim after another node commits the same request and activates its title. It currently returns 409 title_conflict instead of replaying the durable create receipt.

On that title-conflict path only, reread the deterministic dashboard receipt and use the existing replay helper. Matching requests return current configuration and the original applied revision, including after a later edit. Different arguments retain idempotency_conflict; a distinct key retains title_conflict. The retry does not rewrite the document or title.

Stack: depends on prerequisite draft #51; base is dashboard/prerequisite-agent-api (54e8de3352e3804586952f90b3c02aa4a0345804), not main. Only src/users/dashboard_service.rs changes in this layer: the exact supplied fix and regression (158 additions / 3 deletions). Keep draft until the prerequisite's public API compatibility/security/resource-cost blockers and remaining validation are resolved. No broader compatibility claim is made here.

The regression pauses an actual absent read while another service commits, then covers four cases: identical create; identical create after a later edit; same key/different arguments; distinct key/same title. Each checks one document and unchanged stored document/title versions after the late replay or conflict.

Fresh verification: exact patch applicability and reverse check; one-file stack diff; baseline SHA-256 ef366d16c2e8d178b16cbf80a504708e70304feb253adefcb76c89e0bed3a38b; resulting SHA-256 7686f9e862c8b27f6db9d63bdfb73dbcbabcf8707f4c30014f6555ad82f1c8c1; /home/ajs/.cargo/bin/rustfmt --check --edition 2024 --config skip_children=true src/users/dashboard_service.rs; git diff --check: all passed.

Not freshly executed: Rust compilation/test suite and this four-case regression; current CI pending. Historical R81 reconstructed-overlay evidence (835 passed / 1 ignored and the four race subcases) is not a fresh branch test. The prerequisite's fresh 62-test frontend run, TypeScript, formatting and Python syntax checks are documented in #51; the race layer does not change frontend code. No new Rust build target, server, benchmark, dependency installation, merge, or deployment was performed.

---

^{Need help on this PR? Tag @codesmith-bot with what you need.}