Add native Python symbol resolution

[jonathan343]

Note

Stacked on #814, targeting feat/codegen-selection-arena so this PR contains only the symbol-provider work and a small identifier-parser fix.

Summary

Adds SymbolProvider to turn selected Smithy shapes into Python declaration names, field/constant names, and immutable type references. It applies service renames, preserves acronyms, reports naming collisions, and handles recursive types and sparse collections. Enum values resolve to str/int to allow future service values.

Also fixes the loader rejecting valid identifiers such as _2HTTPServer, verified against Smithy CLI. The API and examples are in designs/codegen/symbols.md.

Example

Given a loaded model and its selection, with an HTTPResponse structure containing a statusCode integer field:

from smithy_python.symbols import SymbolProvider

symbols = SymbolProvider(model, selection, package="example.client")

symbols.declaration_name("example#HTTPResponse")        # "HTTPResponse"
symbols.member_name("example#HTTPResponse$statusCode")  # "status_code"
symbols.type_reference("smithy.api#Integer")            # TypeReference("int", "builtins")

Additional Testing

All 434 models in aws/api-models-aws passed independent checks of names, type references, nested arguments, module identity, and sparse nullability: 119,324 type references and 232,207 field targets checked. The 61 streaming references produced the expected unsupported diagnostic. These models contain no service renames; targeted tests cover those. This validation is local, not a CI dependency.

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[kuhe]

these could be aligned using a markdown formatter for reading source mode

[kuhe]

will dynamic client generation be from this loaded model?

[kuhe]

primitive in what sense?

some of these are not fixed-length data, like string, int, big numbers, document.

if this is the set of smithy simple types, it should be called that instead.

[kuhe]

maybe _FIXED_TYPES? not the smithy simple set, but types that have a non-composite/generic python representation