Add BCH helper functions and non-spendable library imports

[lightswarm124]

This PR adds BCH function support on top of current next by introducing contract-local helper functions and non-spendable imported helper libraries. It treats OP_DEFINE / OP_INVOKE as first-class compiler outputs, keeps spend surfaces explicit, and adds the debug/runtime metadata needed to make helper execution reviewable.

Main changes:

• Adds public and internal function support inside contract.
• Adds first-class library containers for imported helper-only code.
• Supports contract -> library and transitive library -> library imports.
• Canonicalizes shared transitive libraries by resolved file identity to avoid duplicate shared-leaf codegen.
• Adds helper-aware debug metadata and SDK attribution for nested helper logs and failures.
• Rejects unsafe helper patterns:
  • recursion and mutual recursion
  • signature-check builtins in helpers
  • contract-constructor capture in helpers
  • imported contract files
  • public functions inside library

Why this matters:

• Enables reusable BCH helper functions both locally and across imported helper libraries.
• Keeps imported helper files non-spendable by construction.
• Makes nested helper execution inspectable enough for maintainers and downstream teams to audit.

Examples included:

Local internal helpers:

• examples/helper-functions.cash
• examples/helper-functions.ts

Imported helper libraries:

• examples/imported-helper-functions.cash
• examples/math-helpers.cash
• examples/parity-helpers.cash
• examples/imported-helper-functions.ts

Testing-suite helper example:

• examples/testing-suite/contracts/internal_functions.cash
• examples/testing-suite/test/internal_functions.test.ts

Docs for review:

• docs/proposals/bch-functions-team-review.md
• docs/proposals/functions-vs-next-review.md
• docs/proposals/functions-implementation-spec.md
• docs/proposals/functions-security-and-release-review.md