test(plugins): reproduce Postgres plugin-storage query/count/index/order bugs (red baseline for #1)

[vedanshujain]

Why this PR is first in the stack

_plugin_storage.data is a plain text column, and plugin storage had only ever
been exercised on SQLite. On Postgres the query / count / index / order paths build
the JSON accessor as an uncast data ->> 'field', which is broken four distinct
ways. This PR adds a dialect-parametric regression suite (wired through
describeEachDialect) that pins each failure mode, so reviewers can see the bug
reproduced before the fix.

This PR is the red baseline. The Postgres dialect fails here on purpose; the
stacked fix (#1) turns it green. SQLite passes throughout, because json_extract
already returns a typed value — which is exactly why the bug went unnoticed.

Failure modes covered

• (a) numeric RangeFilter across a multi-digit boundary — store 9 / 10 / 100,
  query { stock: { gte: 10 } }. '9' >= '10' is TRUE lexically, so 9 leaks into
  the result — an over-count / oversell. Asserts exactly {10, 100}.
• (b) count({ stock: { gte: 10 } }) — asserts 2, not the lexical 3.
• (c) createStorageIndexes UNIQUE expression index over data ->> 'sku' — on
  Postgres this raises operator does not exist: text ->> unknown. Asserts
  errors: [], index created, duplicate insert rejected.
• (d) numeric orderBy — asserts [9, 10, 100], not the lexical [10, 100, 9].

Evidence (EMDASH_TEST_PG set → local urumi-pg-test on 127.0.0.1:55432)

On this branch (no fix) — RED on Postgres, GREEN on SQLite:

tests/.../storage-postgres-regression.test.ts (8 tests | 4 failed)
  × [postgres] numeric RangeFilter { gte: 10 } ...   operator does not exist: text ->> unknown
  × [postgres] count({ stock: { gte: 10 } }) ...
  × [postgres] UNIQUE expression index ...           expected [ { error: "operator does not exist: text ->> unknown", ... } ] to deeply equal []
  × [postgres] orderBy { stock: 'asc' } ...
  Tests  4 failed | 4 passed (8)   # the 4 passing are the [sqlite] variants

With #1 stacked on top — GREEN on both dialects:

  Tests  8 passed (8)   # [sqlite] ×4 + [postgres] ×4

CI note

The Tests job runs a real Postgres service with EMDASH_TEST_PG set and the
default vitest config picks up tests/integration/plugins/**, so the Postgres
job is red on this PR by design. It goes green once #1 lands on top of it. The
Integration Tests job (no Postgres) stays green. This is a stacked, test-first PR
— it is meant to merge as part of the stack, not standalone.

🤖 Generated with Claude Code

https://claude.ai/code/session_01TX9YciGFRZX9aF2UcQ6rUW

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