hrw4u: Add AST for static analysis and codegen (#13126)

* Add AST node dataclasses for hrw4u linter

Frozen dataclasses representing the semantic AST that a
visitor produces from the ANTLR parse tree. Includes Target
decomposition (namespace/field/modifier), all statement nodes
(Assignment, FunctionCall, BreakStatement, StandaloneOperator),
condition expression nodes (Comparison, LogicalOp, Negation),
control flow (IfBlock, ElifBranch), and top-level constructs
(VarDecl, UseDecl, ProcedureDecl, Section).

Type aliases ConditionExpr, BodyNode, and TopLevelNode provide
convenience unions. Tests cover Target.from_dotted parsing, node
construction, and immutability.

* Add AST visitor to build AST from ANTLR parse tree

ASTVisitor walks the ANTLR parse tree and produces HRW4UAST.
Handles named sections, assignments (= and +=), function calls,
break statements, standalone operators, condition expressions
(comparisons, logical operators, negation, set membership, IP
ranges, WITH modifiers), if/elif/else blocks with arbitrary
nesting, and top-level var/use/procedure declarations.

Only visitProgram is overridden from the ANTLR visitor base
class; all other dispatch is internal, keeping the public API
surface minimal. Raises ValueError for unhandled grammar
alternatives to surface visitor-grammar drift early.

Makefile updated to include ast_nodes.py and ast_visitor.py
in the build copy step.

* Add AST visitor tests

Integration tests covering the full visitor pipeline from
source text to AST nodes. Tests are organized by concern:
sections and simple statements, condition expressions (all
operators, logical combinators, negation, parenthesized
grouping), if/elif/else blocks with nesting, real config
patterns (nested conditionals, boolean coercion, IP ranges,
set membership with modifiers, exact match patterns), line
number tracking across all 17 node types, and error handling
for unhandled grammar alternatives.

* Add tagged Value type to AST for codegen support

Introduce ValueKind enum and Value dataclass to preserve
semantic distinction between string literals, identifiers,
param refs, IPs, and regexes in the AST. Without this, the
codegen visitor cannot re-emit values correctly since
_extract_value was collapsing all string-like values into
bare Python str.

* Handle commentLine explicitly in visitProgram

Skip comments intentionally and raise on unrecognized
programItem alternatives to catch visitor/grammar drift.

* Replace tagged Value type with concrete value types

Split Value/ValueKind into LiteralStringValue, IdentValue,
IPValue, ParamRef, and RegexValue so the type system encodes
what kind of value each node carries. This makes the AST more
precise — e.g. iprange is now tuple[IPValue, ...] instead of
tuple[Value, ...], and RegexValue only appears in
Comparison.right where the grammar allows it.

* Raise on unhandled grammar alternatives in AST visitor

Add explicit commentLine handling and ValueError raises
in _visit_var_section, _visit_body, and _extract_value
so new grammar alternatives fail fast instead of being
silently dropped or misclassified.

* Format code

* Add tests for comment skipping and modifier casing

Cover comment handling in section bodies, blocks, and
var sections. Add test confirming modifiers preserve
source casing rather than normalizing.

* Add __all__ to ast_nodes.py for safe wildcard imports

Defines the public API surface explicitly, enabling shorter
`from hrw4u.ast_nodes import *` in downstream modules.

* Use wildcard imports from ast_nodes

Now that __all__ is defined, switch ast_visitor.py and
test_ast_visitor.py to `from hrw4u.ast_nodes import *`.

* Add return type annotations to ASTVisitor methods

Matches the convention used in visitor.py where all visit/helper
methods declare their return types.

* Collapse short constructor calls to single lines

Fits within the project's 132-char column limit per .style.yapf.