Composite/array-valued PK tables: secondary-index lookup by an array value returns 0 rows (breaks @relationship to/from them)

Tables keyed by a **composite / array-valued primary key** (`["tenantA","rec1"]` — the documented composite-key pattern) work for range/prefix scans and ordering, but a **secondary-index lookup by an array value returns 0 rows**, which makes composite-PK tables unusable as `@relationship` targets.

**The defect:** `search_by_conditions` `equals` on an `Any @indexed` field holding an **array** value returns **0 rows** despite matching records existing — the secondary-index path doesn't match array-valued index keys. Consequences:
1. A `@relationship(to:)` **reverse** expansion over a composite-PK target returns **0 refs silently** (looks like "no relations", not an error — the dangerous leg).
2. **Forward** expansion via an array-valued FK 500s (`TypeError ...reading 'then'` at `Resource.ts:808` — the same crash as #1415).

**Correct/clean:** `search_type:'prefix'` on the first component works (the high-value per-tenant range scan); numeric 2nd-component ordering is numeric (not string-coerced); boundary cases round-trip.

**Secondary:** an ops `insert` of a **duplicate composite PK** silently overwrites (200) rather than 409-ing as a scalar PK does.

**Severity:** medium — composite-PK + relationships is a real (if not ubiquitous) combination; the silent-empty reverse expansion is the dangerous part.

**Fix:** fix secondary-index equality matching for array-valued index keys (so `search_by_value`/conditions and `@relationship` reverse-expansion find composite-PK rows). The forward-expansion 500 is #1415.

**Also (doc):** REST addressing of a composite PK needs `encodeURIComponent(JSON.stringify(pk))`, and an ops-seeded composite-PK row is not REST-GETtable.

**Repro:** `npm run test:integration -- "integrationTests/qa-scratch/qa348-composite-pk.test.ts"`

— filed by KrAIs (Claude) on Kris's behalf (qa-explorer campaign, main @ 28db4fde4 v5.1.12)

Start by running npm run test:integration -- "integrationTests/qa-scratch/qa348-composite-pk.test.ts" and trace search_by_conditions equality through the secondary-index path. Inspect the array-valued lookup behavior and the relationship expansion involving Resource.ts:808; done means matching composite-PK rows are found by secondary-index equality and the reverse relationship expansion no longer returns zero rows silently.

Written by the indexing model from the issue text.