[Anand Krishnamoorthi (anakrish)]

Scope

First independent extraction from #934, based directly on main. Related to #932; this preparatory PR does not fix or close that issue.

This PR contains no rule-error propagation, conflict-detection, comparison-budget, compiler, or bytecode-format changes. #934 remains unchanged.

Changes

• Reuse Value::diagnostic() for VM errors, execution states, suspension reasons, and the affected aggregate/set/time builtin errors, avoiding expansion of shared values.
• Add a reusable streaming formatter that limits owned error strings to 256 UTF-8 bytes plus ...; bound generic anyhow-to-VM conversion and affected network parse-error messages.
• Borrow stored execution states when inspecting them or rejecting invalid resumes, rather than cloning potentially large error payloads.
• Preserve full HostAwait payload access independently of diagnostic text: add Python/Java/WASM compact-JSON accessors and C++/Go wrappers around the existing C accessors.
• Add shared-value, large-string, UTF-8, strict/non-strict, execution-mode, FFI, and binding regressions; update four YAML diagnostic expectations.

Structured Rust values remain intact. Debug text intentionally changes and is not a payload serialization API. These changes cover VM diagnostic formatting and the builtin producers touched here, not every third-party error constructor.

Validation — debug builds only

• cargo xtask ci-debug --frozen — passed, including OPA conformance and feature combinations.
• All-feature diagnostic unit/integration tests — passed.
• cargo xtask test-no-std --frozen — passed for thumbv7m-none-eabi.
• FFI suite — 78 passed, run serially.
• Python, Java, WASM/Node, Go RVM, and C++ RVM smoke tests — passed against debug libraries; WASM used --dev --no-opt, C++ used -O0.
• cargo xtask fmt --check and git diff --check — passed.

Builds used at most two Rust jobs. The installed toolchain needs --force-warn deprecated_in_future for two existing Azure builtin deprecations. Strict Clippy also encounters five existing findings in unchanged files (question_mark, for_kv_map, and useless_borrows_in_formatting); the complete workspace/binding Clippy task is checked with only those categories downgraded to warnings. No repository lint settings are changed.