Add Partial Result Cache for repeated nested-loop and correlated subquery execution

Problem

Repeated outer or dependent parameter values can make nested-loop joins and
correlated subqueries execute the same inner plan many times within one
statement. This is wasted work when the inner result for a parameter value is
deterministic and can be reused safely.

Proposed solution

Add a statement-local Partial Result Cache, also called PTRC, above eligible inner work:

• nested-loop join inner paths
• deterministic dependent subqueries

The cache is local to a single statement. It is not shared across statements,
sessions, or transactions, and it does not change table data visibility.

The implementation adds:

• a new access path / iterator for partial result cache execution
• optimizer_switch=partial_result_cache
• PRC_SUBQUERY / NO_PRC_SUBQUERY and PRC_JOIN / NO_PRC_JOIN optimizer hints
• bounded memory controls for statement-local cache storage
• LRU eviction by complete key batch
• pass-through bypass mode when caching is unsupported, not useful, or cannot
  initialize
• EXPLAIN / EXPLAIN ANALYZE / optimizer trace diagnostics

Unsupported or fallback cases

Unsupported shapes keep the original plan or enter pass-through bypass mode.
Examples include non-deterministic expressions, unsupported dependent subquery
shapes, BLOB/TEXT cache keys, const-only paths, and low estimated or runtime
hit-ratio cases.

Correctness

The cache preserves scalar, EXISTS, IN, ANY, and ALL correlated-subquery
semantics by storing result item state, no-match state, and NULL state where
needed.

Validation

The current PTRC branch has focused MTR coverage for hints, nested-loop joins,
correlated subqueries, diagnostics, acceptance behavior, semantics, error paths,
fault injection, and follow-up regression cases.

Full MTR integration validation was run during the MySQL 8.0.46 PTRC port.
PTRC-visible expected-result changes were reconciled. Known local non-PTRC
environment failures were tracked separately.

Local performance validation on repeated TPC-H Q17 workloads showed material
improvement with identical query output hashes. These numbers are engineering
evidence only, not official TPC-H results.

Implementation branch / pull request

Initial development branch:
https://github.com/ZhuQingping/mysql-server/tree/support_partial_result_cache_8.0

Licensing

I submit this contribution under the New BSD License, also known as the
3-clause BSD License.