feat(pgpm): reuse — expand a proxy into shared + per-tenant modules

[pyramation]

Summary

Makes a source pgpm package reusable across tenants while emitting objects the classifier proves are tenant-independent once. Builds on the splitBundle primitive (originally this PR) by wiring the partitioner (pgpm/slice) into apply.

A pgpm.apply.json may now declare a reuse split:

{
  "source": "catalog",
  "schemas": { "catalog": "tenant_a" },
  "reuse": {
    "sharedSchema": { "catalog": "catalog_shared" },
    "perTenant": [{ "fromSchema": "catalog", "kind": "table", "name": "products" }]
  }
}

The proxy expands into two ordinary pgpm modules — no new deploy magic, just modules + requires:

• a shared module (objects reachable from no seed, e.g. catalog_shared.slugify), deployed once;
• a per-tenant module (a seed or anything transitively reaching one, e.g. tenant_a.products + tenant_a.product_slug) that requires the shared one.

How it works (apply/reuse.ts)

bundleFromModule(source)
  → partitionModule({ seedObjects })          // classifier: which changes are per-tenant
  → makeSchemaTranspiler({                     // ONE object-routed transpile:
      schemaMap: reuse.sharedSchema,           //   default → shared schema
      routes: perTenantChanges → tenant schema //   per-object override → tenant schema
    })                                         //   so product_slug lands in tenant_a but
                                               //   still calls catalog_shared.slugify
  → splitBundle(transpiled, {
      perTenantChanges,
      sharedName, perTenantName,
      perTenantBootstrap: [tenant CREATE SCHEMA]  // each tenant owns its own schema;
    })                                            // per-tenant deps re-point off shared schema
  → verifyBundle + materializeBundle (both halves)

Cross-boundary references stay correct because a single SchemaRouter knows both destinations: the per-tenant function is defined in tenant_a yet references tenant_a.products (local) and catalog_shared.slugify (cross-module requires).

Deterministic shared identity / dedup

resolveSharedModuleName(spec) hashes { source, sharedSchema, seeds } → "<source>-shared-<hash8>" (overridable via reuse.sharedName). Two tenant proxies over the same source/shared/seed config resolve to the same shared module, so addApplyModules adds it once and both tenants require it → deployed once, and reverting one tenant leaves the shared state (still needed by the other) intact. resolveEffectiveModulePath returns the shared or per-tenant materialization by requested name.

Validation (apply-spec.ts)

reuse must be an object with a non-empty sharedSchema string→string map and a non-empty perTenant seed array ({ fromSchema, kind, name }, kind ∈ table|view|function|procedure|type). Every seed schema must have a target in both schemas (per-tenant) and reuse.sharedSchema (shared). Existing schemas/route/requires behavior is unchanged.

Tests

• apply-reuse.test.ts:
  • spec parsing/validation (valid + malformed reuse, deterministic + overridden shared name, cross-tenant name equality);
  • fs-level materializeReuseModule (shared helper emitted once; table + dependent fn routed per tenant; cross-module dep on the shared helper; local schema bootstrap; idempotent CREATE SCHEMA IF NOT EXISTS);
  • two-tenant DB e2e via the pgpm/pgsql-test harness: tenant A schema absent, tenant B schema pre-existing; shared helper deployed once; table + tenant-reading fn materialize per tenant; the fn depending on the tenant table is not shared; verify passes for both; reverting the newest tenant leaves the shared helper + the other tenant intact.
• split.test.ts: perTenantBootstrap coverage (schema injected/prepended, dependency + SQL-header rewrite, collision + non-shared-replace rejection).

Full suites green: core (388), bundle (44), slice (40); core typecheck + build clean.

Planning: constructive-planning#1298.

Link to Devin session: https://app.devin.ai/sessions/025fb88043964fdbb335ac5e39df2478
Requested by: @pyramation

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