refactor(db): move reaction store ownership

[TheSentinel454]

Current reconstructed head

Exact base: codex/issue-2-thread-store at 15bf0e76286540a4102193b70a7058735df79f60
Exact head: codex/issue-2-reaction-store at e2c71f5df1647e8a583ed3070c01fe7eb2bb85b3

This current head removes crates/buzz-db/tests/store_ownership.rs; no replacement path-sensitive ownership test is introduced. Apart from removing that complete test-file diff, the production patch is byte-for-byte identical to the previously reviewed slice. This remains part of tracker #2 and the #17/#19 acceptance work.

Independent exact-head review from a separate clean Blox workstation found no issues. Current-head evidence passed formatting, strict buzz-db clippy, 111 non-PostgreSQL library tests with 200 PostgreSQL tests ignored, the observability source test, relay consumer compilation, exact ownership/unique-span review checks, and 6 reaction PostgreSQL tests on native PostgreSQL where applicable.

Why

Continue tracker #2 by giving reaction persistence one owner while preserving the event-transaction boundary established by the parent slices. This child stacks on #6789.

What

• Move all eight reaction-facing Db APIs and datastore spans into reaction.rs
• Move ReactionEventInsertOutcome, the atomic target/reaction/event transaction orchestration, its five focused PostgreSQL tests, and the cross-community reaction test into reaction.rs
• Preserve buzz_db::ReactionEventInsertOutcome and the existing buzz_db::event::{ReactionEventInsertOutcome, insert_reaction_event_with_thread_metadata} compatibility paths
• Keep the general insert_event_with_thread_metadata_tx primitive in event.rs; the reaction owner composes it with add_reaction_tx without exposing a raw pool or broad executor API
• Preserve mention materialization after a newly inserted reaction event, duplicate short-circuiting, rollback, reactivation, target-community isolation, SQL/binds, and exactly one datastore span per public logical operation

Stack

• Exact base: codex/issue-2-thread-store at d43426e (#6789)
• Exact head: codex/issue-2-reaction-store at 19f2b86
• Tracker: buzz-db: break up lib.rs — move impl Db methods to domain modules TheSentinel454/buzz#2
• Domain issue: none; this is a remaining reaction-domain slice identified by the tracker ledger
• Test/span acceptance: buzz-db: move lib.rs tests (~4,000 lines) to domain modules TheSentinel454/buzz#17 and buzz-db: consistent #[datastore_span] tracing placement TheSentinel454/buzz#19

Non-goals

• No SQL, schema, locking, transaction ordering, retry, timeout, pagination, or client-visible behavior changes
• No movement of the general event insertion transaction primitive or mention materialization into a reaction-only abstraction
• No store traits, domain-handle redesign, broad PgExecutor migration, raw pool accessor, new crate, or directory-wide reorganization
• No changes to, retargeting of, or merge action on parent PRs or PR Add database pressure observability #6700

Risk Assessment

Moderate review surface, low semantic risk. The reaction SQL and Db bodies moved intact; the atomic kind:7 path keeps its target lookup, reaction upsert, duplicate rollback, event insert, commit, and post-commit mention behavior in the same order. Module qualifications and compatibility re-exports are the only API-path changes. Six PostgreSQL tests cover the transaction and tenant invariants, and seven retained event PostgreSQL tests guard the shared insertion primitive.

Blox Verification

Author workstation: buzz-tornquist-issue-2-store-stack (2046520), exact head f614cc212c70c91aacd5e4013c0dc77f847bce8a.

• cargo fmt --all --check and git diff --check — passed
• cargo clippy -p buzz-db -p buzz-relay --all-targets -- -D warnings — passed
• Native PostgreSQL reaction::tests:: ignored suite — 6 passed
• Native PostgreSQL retained event::tests:: ignored suite — 7 passed
• cargo test -p buzz-relay --lib -- --test-threads=1 — final rerun 909 passed, 48 ignored. An initial run had the known unrelated mesh-demo 504 timeout (908 passed, 1 failed); the exact failed test passed immediately before the full green rerun
• Commit-time formatting, sadscan, attribution, and sign-off hooks — passed

No findings at any severity. The clean detached review workstation was deleted after its artifacts were verified locally.

Generated with Codex

Superseded pre-comment restack verification

PR #6700 merged before publication completed. This layer was restacked onto current main through the exact parent named above; the final cumulative tip is 2ddcc8a. Cumulative author gates passed: formatting and diff checks; buzz-db and buzz-relay all-target clippy with -D warnings; DB lib 111 passed / 200 ignored; ownership 22/22; observability 1/1; the full isolated PostgreSQL domain matrix; and relay lib 910 passed / 49 ignored.

• Workstation: buzz-tornquist-pr-6792-final-review (2057619), fresh shallow checkout
• Base: a03619b3f5e2c981de704be13f609f117eaf6fa1
• Head: b87cec6d690119721e2ef3ac34e6ba57b41099c7
• Findings: none

Reviewed reaction records, SQL, transaction orchestration, Db wrappers, tests, and spans moving to reaction.rs. The general event transaction primitive remains event-owned, while compatibility re-exports preserve prior event-module and crate-root outcome paths. Duplicate short-circuit, cross-community target rejection, soft-delete reactivation, event/thread/reaction atomicity, and public signatures are unchanged; each logical public operation retains one datastore span.

Verification: format and diff checks passed; buzz-db --all-targets clippy passed with -D warnings; DB lib tests passed (111 passed, 200 PostgreSQL tests ignored); ownership (8/8) and observability (1/1) guards passed; all 6 reaction PostgreSQL tests passed on native PostgreSQL 17 with migrations 1-32 successful; relay lib test target compiled successfully. Final worktree was detached at the exact head and clean.

Complete evidence archive SHA-256: 471754cd65139b3494a9efdaf5528505fa8928b092d055e6af3560e532a32183.

Comment-addressed restack

Review follow-up on #6777 removed only the low-value replaceable ownership source test. This PR was restacked onto its rewritten parent; its production patch is unchanged.

• Exact base: d43426ed89b89d49e44a75f4a4e1e57f4915c087
• Exact head: 19f2b86a063cecd1521eb003225ec76acb5d8bf3
• Final cumulative tip: 6fa2f104d42c6ba85bdf62e7ccb74ceaf4a84f67
• Per-layer patch-ID and tree audits confirm this PR’s production diff is unchanged from its pre-comment head.
• Cumulative Blox gate: formatting and diff checks; strict buzz-db/buzz-relay Clippy; DB lib 111 passed / 200 ignored; ownership 21/21; observability 1/1; every moved PostgreSQL test; relay lib 910 passed / 49 ignored.
• Independent re-review at this exact head: no findings; fresh exact-parent/head Blox review passed fmt/diff, strict Clippy, DB lib 111 passed / 200 ignored, current ownership/observability guards, 6 reaction PostgreSQL tests, and relay compilation.