Add evaluate_expression entrypoint

[jcschaff]

Summary

Adds evaluate_expression: evaluate a native-syntax VCell infix expression against a {symbol: value} table and get a 64-bit float back — or a structured error.

from libvcell import evaluate_expression, VCellExpressionError

evaluate_expression("a + b/c", {"a": 10.0, "b": 20.0, "c": 5.0})  # -> 14.0
evaluate_expression("2 + 3*sqrt(4)", {})                          # -> 8.0

try:
    evaluate_expression("1/c", {"c": 0.0})
except VCellExpressionError as e:
    e.error_type   # "DivideByZeroException"

Design

• Structured boundary object EvalReturnValue(success, value, error_type, message) mirrors the existing ReturnValue. The native entry point always returns a JSON document; Python parses it into EvalReturnValue.
• Two layers: native_calls.evaluate_expression returns the raw EvalReturnValue (branch on .success); the public model_utils.evaluate_expression returns the float or raises VCellExpressionError (carrying .error_type + .message).
• Evaluation uses vcell-core's Expression: parse → bindExpression(new SimpleSymbolTable(names)) → evaluateVector(values). SimpleSymbolTable gives the lightweight dummy binding — no VCell model / MathDescription needed. Any symbol the expression references must be supplied; extras are ignored.
• Non-finite results (Inf/NaN, unrepresentable in JSON) are surfaced as an error.

Changes

Java (vcell-native/): ModelUtils.evaluateExpression, Entrypoints.evaluateExpression @CEntryPoint (JSON in/out), MainRecorder exercise, 7 JVM tests.
Python (libvcell/): native_utils (hasattr-guarded registration), native_calls (EvalReturnValue + call), model_utils (evaluate_expression + VCellExpressionError), exported from __init__; Python tests.
Docs: new docs/design.md (architecture, FFI conventions, entry-point catalog, evaluate_expression, how-to-add-an-entrypoint) + mkdocs nav + API reference. Also ignore stray uv.lock (project uses Poetry).

Verification

• Java: 30/30 tests pass (7 new — value, constant, extra-symbols, unbound → ExpressionBindingException, parse error, divide-by-zero → DivideByZeroException, domain → FunctionDomainException).
• Local native rebuild + full Python suite 19/19 pass (the 8 evaluate_expression tests run against the rebuilt dylib, incl. pytest.raises(VCellExpressionError) with expected error_type).
• make check green (ruff, prettier incl. docs, mypy strict, deptry).

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