feat(sql): generate-vs-hand-maintain follow-up for the function catalog (#225)

[zzylol]

Summary

Part of #225 -- completes item 3 of its "Proposed direction" ("Where
feasible, generate rather than hand-maintain"), which #227 deliberately left
undone. Stacked on top of #227 (feat/sql-function-catalog-225): this PR's
base is that branch, not main, since asap-sql-function-catalog doesn't
exist on main yet.

Issue #225 names two introspectable sources and treats them differently:

1. DataFusion's own aggregate-function registry -- introspectable
   in-process, no external dependency, no infra decision needed.
2. ClickHouse's system.functions -- needs a live/embedded ClickHouse,
   which this repo has no CI infra for.

1. DataFusion registry drift detection (real, CI-exercised Rust)

crates/frontend-sql/src/sql/mod.rs gets two new tests (mod catalog_drift)
that build a real SessionContext the same way SqlLowerer::build_context()
does and:

• assert every aggregate name it resolves is covered by
  asap_sql_function_catalog::NATIVE_FUNCTIONS, is one of the catalog's own
  CLICKHOUSE_BUILTINS stub registrations, or is on a new, documented
  KNOWN_UNMAPPED_NATIVE_FUNCTIONS allow-list (each entry has a reason:
  no AggIntent shape yet, or explicitly rejected elsewhere);
• assert every KNOWN_UNMAPPED_NATIVE_FUNCTIONS entry is still a real
  DataFusion name (so the allow-list itself can't quietly rot).

This turns a future DataFusion version bump that adds/renames/removes a
builtin aggregate into a cargo test failure instead of silent catalog
drift -- no new CI infra, it's part of the existing cargo test --workspace.
I verified the test actually catches drift by temporarily removing an entry
from the allow-list and confirming it fails with a clear message, then
restored it.

Along the way this surfaced two small, pre-existing gaps that the old
hand-written match never had a way to reveal: DataFusion resolves
var_sample / var_population (its own alias spellings of var_samp /
var_pop) but NATIVE_FUNCTIONS doesn't list those spellings. Rather than
silently "fixing" that in this PR, they're recorded on
KNOWN_UNMAPPED_NATIVE_FUNCTIONS with a comment -- accepting a new spelling
is a maintainer's call, and surfacing it rather than deciding it is the
point of this tooling.

2. ClickHouse builtin discovery (dev-only, not CI-wired)

tools/clickhouse/extract_functions.py diffs ClickHouse's own
system.functions (aggregate functions only) against
CLICKHOUSE_BUILTINS and reports:

• candidates -- ClickHouse aggregate names not yet in the catalog,
• possibly stale -- catalog entries ClickHouse's list no longer has,
• combinator-derived -- catalog entries like countif that are really
  a base function (count) plus one of ClickHouse's aggregate combinators
  (-If, -Distinct, -Array, ...), which ClickHouse doesn't enumerate as
  its own system.functions row -- so a naive diff would misreport these as
  stale every run; they're called out separately instead.

It uses chdb (ClickHouse embedded
in-process) rather than a live server -- pip install chdb is the only
setup step, no Docker, no network service. tools/clickhouse/README.md
covers how to run it, and explicitly documents why it's not wired into
CI (no ClickHouse service anywhere in this repo's CI, and that's a
deliberate decision, not an oversight).

It never touches crates/sql-function-catalog/src/lib.rs -- it's a
reporting tool for a human to act on, matching the issue's own scope
(existence + arity only; RewriteKind/semantic is a human judgment call).

Explicitly out of scope (per the issue and this follow-up's own scope)

• No CI/workflow changes of any kind.
• No auto-generation of RewriteKind/AggSemantic assignments.
• ElasticSQL untouched (separate, bigger gap per SQL: extract dialect builtin-function catalog into independent, generatable data (inspired by polyglot-sql-function-catalogs) #225).

Test plan

• cargo build --workspace --all-targets
• cargo test --workspace (all green, including the two new
  catalog_drift tests)
• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• Manually verified the drift test fails with a clear message when a
  catalog entry is removed, confirming it actually detects drift
• tools/clickhouse/extract_functions.py run end-to-end against real
  embedded ClickHouse via chdb (installed successfully in this
  sandbox) -- produced a correct, sensible diff report, including
  correctly classifying countif as combinator-derived rather than
  falsely "stale"
• python3 -m unittest tools/clickhouse/test_extract_functions.py
  (16 tests, all passing) -- covers the catalog-parsing and diff logic
  without needing ClickHouse at all

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