test(db): seed canonical large roster fixture

[TheSentinel454]

Summary

Make the 11,000-member feed mention-indexing regression fixture canonical under migration 0032 before the feed store extraction adds more ownership to feed.rs.

The test now seeds the matching channel_members rows and emits four-field kind-39002 p tags with the authoritative member role. Its bind-parameter-cap assertion and 11,000-row scale are unchanged.

Stack

• Exact base: codex/issue-2-reaction-store at 19f2b86 (#6792)
• Exact head: codex/issue-2-feed-roster-test-fixtures at f18b0d3
• Structural tracker: buzz-db: break up lib.rs — move impl Db methods to domain modules TheSentinel454/buzz#2
• Domain issue: none; test-only prerequisite for the feed slice
• 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
• Next: codex/issue-2-feed-store

Why separate

The existing ignored test is red against a freshly migrated current main: migration 0032 rejects its legacy two-field roster tags before insert_mentions is exercised. The feed extraction PR is intended to move ownership without behavior changes, so this fixture correction is isolated in its own reviewable commit.

Non-goals

• No production Rust or SQL changes.
• No schema, lock, transaction, timeout, retry, API, or client-visible behavior changes.
• No change to mention-index insertion or batching.
• No feed store ownership movement; that remains in the child PR.

Risk

Low and test-only. The fixture still creates 11,000 unique mentions, exceeds the single-statement bind cap at six binds per row, and asserts every mention is indexed.

Blox verification

Author workstation: buzz-tornquist-issue-2-store-stack (2046520), native PostgreSQL 17.11.

• Red before the change on a freshly migrated database: the focused test failed with migration 0032's kind 39002 roster contains an invalid p tag constraint.
• cargo fmt --all --check: pass
• git diff --check: pass
• cargo clippy -p buzz-db --all-targets -- -D warnings: pass
• feed::tests::insert_mentions_indexes_rosters_past_bind_parameter_cap: pass
• Cumulative exact-tip PostgreSQL matrix: pass

Superseded pre-comment restack verification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.

Outcome

No actionable findings.

Reviewed the exact range b87cec6d690119721e2ef3ac34e6ba57b41099c7..ed21d81e7fe1fa7606c6e67412b788cc6df12916 on fresh Blox workstation buzz-tornquist-pr-6820-final-review (ID 2057635). The head is the direct child of the exact base. The test-only change seeds channel_members with the same 11,000 deterministic pubkeys emitted in four-field canonical roster p tags; it restores the large-roster bind-cap regression without changing production behavior.

Verification

• cargo fmt --all --check, range/worktree git diff --check, and clean-tree checks passed.
• cargo clippy -p buzz-db --all-targets -- -D warnings passed.
• Store-ownership guards: 8 passed; observability source guard: 1 passed.
• Non-PostgreSQL feed tests: 22 passed, 4 ignored.
• cargo test -p buzz-relay --lib --no-run passed.
• Native PostgreSQL 17: feed::tests::insert_mentions_indexes_rosters_past_bind_parameter_cap passed with the current migrations.
• Final detached HEAD and merge-base were rechecked; the repository remained clean.

No code edits, commits, pushes, comments, or PR mutations were made. Buzz relay credentials were unavailable, so the private guide, ownership report, and workstream-channel updates were unavailable under the documented fallback. The workstation was deleted after artifacts were transferred.

Artifacts

• Diff SHA-256: 4df56ed54eb6b5bba06fd26354e1937b40fddf785759dce475b10de6c67e4ad6
• Verification log SHA-256: 931f2a86c0aaf54f51c1c70a6df0b359a759cdd40ceec22684628e679c72d806
• Artifact archive SHA-256: 481b8bd4a98e1db2da8b2425f981109cbcaced6d11960083703ee1a4eed229ac

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: 19f2b86a063cecd1521eb003225ec76acb5d8bf3
• Exact head: f18b0d3fdc45610d375637c2585d8ae589762735
• 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 buzz-db Clippy, current ownership/observability guards, feed tests (22 passed / 4 ignored), the moved large-roster PostgreSQL fixture, and relay compilation.