Flips the Python dataflow trunk from the legacy CFG (semmle/python/Flow.qll)
and legacy ESSA SSA (semmle/python/essa/*) to the new shared CFG facade
(semmle.python.controlflow.internal.Cfg) and the new SSA adapter
(semmle.python.dataflow.new.internal.SsaImpl), both introduced
additively in the preceding PRs in this stack.

This is the trunk-flip equivalent of the original draft PR #21894 (kept
around as documentation), rebased on top of the four preparatory PRs:

  P1: Remove AstNode.getAFlowNode() and rewrite callers (#21919).
  P2: Qualify Flow.qll's AST references with Py:: prefix (#21920).
  P3: Add new shared-CFG-backed control flow graph (#21921).
  P4: Add new shared-SSA-backed SSA adapter (#21923).

The Python dataflow library (semmle/python/dataflow/new/) now imports
the new CFG facade and SSA adapter. All CFG-typed predicates
(ControlFlowNode, CallNode, BasicBlock, NameNode, AttrNode, ...) are
qualified with the Cfg:: prefix; SSA references switch from
EssaVariable/EssaDefinition to SsaImpl::Definition/SourceVariable.

GuardNode is redesigned to use the new CFG's outcome-node model
(isAfterTrue / isAfterFalse) instead of the legacy ConditionBlock +
flipped indirection. Only BarrierGuard<...> is preserved as public
API.

Framework files (Bottle, FastApi, Django, Tornado, Pyramid, Stdlib,
...) are updated to take CFG nodes from the new facade.

A handful of dataflow consistency tweaks for the new CFG:
- Augmented-assignment targets are treated as both load and store.
- 'from X import *' produces uncertain SSA writes for unknown names.
- CFG nodes are canonicalised so dataflow does not see equivalent
  pre/post-order pairs as distinct nodes.

Two AST tweaks for the new CFG:
- AstNodeImpl: omit PEP 695 type-parameter names from
  FunctionDefExpr / ClassDefExpr children.
- ImportResolution: drop the legacy essa import.

Test churn (~175 files): reblessed library- and query-test .expected
files reflect slightly different CFG granularity, different toString
output, and a handful of true alert deltas in security queries.

Verification: all 367 lib + src + consistency-queries compile clean.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

The `Cfg::ControlFlowNode` facade re-exports the shared CFG library's
`dominates`/`strictlyDominates` predicates, which are declared
`bindingset[this, that]` + `pragma[inline_late]` and are meant to be used
as bound-pair membership checks. The facade wrappers dropped these
annotations (using plain `pragma[inline]`), so even though the only
callers — the `with` / `async with` taint steps in DataFlowPrivate.qll
and TaintTrackingPrivate.qll — bind both endpoints, the optimizer was
free to materialise `Cfg::ControlFlowNode.strictlyDominates/1` as a full
O(nodes^2) relation over the (larger) shared-CFG node set.

On some projects this dominated analysis time entirely (DCA showed e.g.
ICTU/quality-time and biosimulations regressing ~75-160x). Restoring
`bindingset[this, other]` + `pragma[inline_late]` on the wrappers turns
the predicate back into a bound-pair check and is result-preserving (only
binding annotations change, the predicate body is unchanged).

Reproduced on ICTU/quality-time: full python-security-extended suite went
from stalling >20min on `strictlyDominates` to completing in ~6min; all
ControlFlow and dataflow/coverage library tests pass.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

The legacy CFG (`Flow.qll`) and legacy ESSA (`Essa`/`SsaCompute`/
`SsaDefinitions`) were pinned into the always-on `Stages::AST` cached stage
via `Stages::AST::ref()` and the matching `backref()` disjuncts. Because a
cached stage is materialized as a unit once any of its predicates is demanded
(and every query demands e.g. `Expr.toString()`), this forced the legacy
CFG/ESSA to be computed for *every* query -- including the security/dataflow
queries, which after the shared-CFG dataflow flip no longer depend on the
legacy CFG at all.

Since `Stages::AST::ref()` is `1 = 1`, removing it is result-preserving; it
only changes stage scheduling. After this change the legacy CFG/ESSA is no
longer materialised for queries that do not genuinely reference it. Verified
on the full `python-security-extended` suite and on django: legacy CFG/ESSA
families materialised drop from ~165 to 0 with byte-identical results.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>