Add preprocessor parity and expanded fixture harness support

[rlippmann]

Summary

Adds TypeScript parity support for the Python experimental preprocessor contract and expands TS fixture harness coverage to match the newer Python conformance surface.

This PR:

• syncs TS fixtures through Python v0.6.19

• adds harness support for:

  • conformance/state-json
  • conformance/checkpoint
  • preprocessor

• implements experimental preprocessor parity against the synced Python fixture/API contracts

• moves the preprocessor implementation into a dedicated experimental module (src/experimental/preprocessor)

• removes experimental preprocessor exports from the root TS package surface

Why

TS parity work had expanded beyond the original step/transcript fixture surface.

Python 0.6.18 and 0.6.19 introduced:

• portable serialization/checkpoint fixture categories
• portable preprocessor fixture/API contracts
• cleaned canonical preprocess/preprocessor naming

This PR brings TS into alignment with those authoritative fixture contracts before future TS 0.6 stabilization/release work.

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 tests were run

Notes

• No core engine behavior changes.

• No Python fixture modifications.

• Experimental preprocessor APIs are intentionally isolated from the root TS package exports.

• Package subpath export wiring for experimental modules is intentionally deferred to later packaging/release work.

• Validation run:

  • npm test
  • npm run fixtures:check
  • npm run fixtures:preprocessor:check