feat(membership): refuse episode membership outside the episode

[seonghobae]

A document may belong to an episode only while that episode is active in event time (ADR 0003). Membership cannot start before or end after the episode interval. Recovery is the computed share of containment flags that match known truth versus accepting every membership.

Complementary to #118 (`subevent_containment`: child event versus parent event). This crate owns episode-membership containment.

Local gates:

• `cargo test -p episode_membership --all-targets` GREEN after RED (package did not exist)
• clippy `-D warnings` PASS
• workspace contract PASS
• docstring contract PASS
• lines 20/20; nightly-2026-08-01 branches 12/12

Does not allocate migration `0008`. Does not recreate `stopword_deletion` (#145), `psychometric_core` multilevel (#144), or other in-flight crates.

Keep this PR draft. Preferred merge remains #46 only when exact-head required Checks pass and a qualifying independent (non-Cursor/CodeRabbit) APPROVE exists. Do not empty-commit.

[coderabbitai]

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