chore: prepare 0.7.3 release docs and smoke-test ergonomics

[rlippmann]

Summary

• Bump package version to 0.7.3.
• Improve TS README guidance to:
  • prefer helper-based state reads in Quick Start
  • clarify opaque state snapshot expectations
  • document preprocessor safety boundaries (validation, pending-clarification bypass, conservative behavior)
  • update fixture parity wording to Python source-of-truth corpus
• Improve optional Vercel AI SDK smoke-test ergonomics by adding explicit optional-dependency preflight checks with actionable install instructions.
• Update Vercel structured-output integration README with exact optional install/smoke commands and API key requirement.
• Add camelCase preprocessor option ergonomics support: sourceInput now works alongside existing source_input (backward compatible).

Why

• Final pre-release polish for 0.7.3.
• Keep user-facing TS docs aligned with current camelCase API ergonomics and safety expectations.
• Make opt-in smoke-test failures actionable instead of raw module-resolution errors when optional dependencies are not installed.
• Reduce TS integration friction by supporting sourceInput in camelCase API usage while preserving parity compatibility.

Scope checklist

• Diff is minimal and scoped to the requested behavior
• Python/spec parity is preserved (or mismatch is explicitly documented)
• Fixture semantics are respected (exact string vs non-empty null rule)
• No unrelated refactors
• Relevant validation was run (npm test for behavior changes, or docs-only/no-runtime-change)

Notes

• No core engine/preprocessor semantics changed.
• No fixture JSON files were modified.
• Optional smoke test remains opt-in via CONTEXT_COMPILER_RUN_VERCEL_AI_SMOKE=1.
• source_input remains supported; sourceInput is additive and normalized via sourceInput ?? source_input.

Validation details:

• npm test: passed
• If not run, why: n/a