persistence(membership): enforce unit-interval weight parity with Membership owner #614

Persistence contract finding

membership_core::MembershipWeight on #605 owns affiliation-share semantics and rejects weights above one. TEPP's persistence boundary does not yet preserve that invariant:

• MembershipAssignmentRecord::validate rejects non-finite and <= 0, but accepts finite > 1;
• migration 0006_typed_membership_assignment has CHECK (membership_weight > 0) but no upper bound, so direct SQL can persist 1.25.

Zero is already correctly rejected in persistence and PostgreSQL; #613 aligns the Membership VO to that existing positive-domain contract. This issue covers the distinct upper-bound mismatch.

Single-writer / migration boundary

Do not rewrite historical migration 0006. PR #287 is the active persistence lineage and reserves 0008_analysis_run_persistence. The repair must therefore be an ordinary-forward dependent persistence vehicle based on #287's exact head, allocating 0009 only after inheriting #287's 0008 migration.

Required RED

The dependent persistence vehicle must prove before repair that:

1. MembershipAssignmentRecord rejects 1.0 + f64::EPSILON and other finite >1 values;
2. the database refuses a direct membership_assignment insert above one after the full embedded migration catalog is applied;
3. positive sub-unity values and exact 1.0 remain valid;
4. rollback removes only the new bound and restores the predecessor schema without rewriting earlier migration history.

Repair boundary

• tighten the Rust record validator to finite (0,1];
• add forward migration 0009_membership_weight_unit_interval with a named multiword constraint membership_assignment_weight_unit_interval CHECK (membership_weight > 0 AND membership_weight <= 1) (or equivalent fail-closed form);
• update the embedded migration catalog in sequence after feat(persistence): persist idempotent analysis runs #287's 0008;
• retain RLS, append-only, temporal, tenant and live PostgreSQL contracts;
• no tolerance, clamp or normalization.

Acceptance

Exact-head persistence tests, live PostgreSQL 18.x migration/direct-write/rollback evidence, rustdoc/line+branch+edge coverage, security/CodeQL/Semgrep, and qualifying independent review. Keep this issue open through both #287 prerequisite landing and the dependent persistence PR's protected-main landing.