fix: add controller helper parity accessors

[rlippmann]

Summary

Sync the latest Python public API contract into the TS repo, add controller helper parity accessors, update the CI fixture pin, and refresh TypeScript docs/examples to use the current helper and constant surfaces.

Why

Python added controller helper accessors to the public API contract. TS needed to sync that contract first, implement the matching helper exports without changing controller result shapes or engine semantics, and then align CI and examples with the updated public surface.

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

• Synced the Python conformance/api/public-api-v1.json fixture first so the contract drove the TS implementation.
• Added camelCase controller helpers plus snake_case parity aliases, with snake_case as the primary implementation and camelCase as aliases.
• Updated the CI Python fixture SHA pin to the Python commit used for the synced public API contract.
• Refreshed the controller example and selected README/integration snippets to use the new helpers and existing exported constants where that improves clarity.

Validation details:

• npm test: passed
• If not run, why: n/a
• npm run build: passed
• FIXTURES_SOURCE=/Users/rlippmann/Source/context-compiler/tests/fixtures/conformance npm run fixtures:check: passed
• npm test -- --run tests/examples-smoke.test.ts tests/demos-smoke.test.ts: passed