Guard prototype chain access to prevent prototype pollution

[shepherdwind]

Motivation

• Prevent accidental or malicious traversal of object prototype properties (like __proto__, constructor, prototype) when resolving references and assignments.
• Consolidate prototype-related checks into a single shared module to avoid duplicated logic and make intent explicit.
• Protect property, index and method access paths used by the template runtime to reduce risk of leaking or modifying prototype-shared state.

Description

• Add a new module src/compile/prototype-guard.ts that centralizes prototype-key detection and helpers: hasOwnProperty, isBlockedPrototypeKey, and hasBlockedUnknownPrototypePath.
• Update src/compile/references.ts to import the guard and block dangerous top-level or path accesses, including checks before reading context keys, property lookups, index lookups and method invocation, and throw a TypeError on unsafe method calls.
• Update src/compile/set.ts to use the shared guard helpers and remove duplicated prototype-check logic, including blocking unsafe set paths and guarding unknown path resolution.
• Add unit tests in test/references.test.ts covering prototype chain protection scenarios for top-level references, __proto__ index/property forms, inherited constructor/prototype exposures, preserving own constructor/prototype fields, and function prototype paths.

Testing

• Ran the project's unit test suite with npm test and the test run completed successfully.
• The new prototype chain protection tests in test/references.test.ts passed as part of the suite.

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