S-079: add canonical mixed-form transaction

[airmang]

What changed

• add strict hwpx.mixed-form-plan/v1 and compiled-plan contracts for native fields, label-adjacent cells, canonical paths, and exact-one body anchors
• resolve all targets before mutation and compile one existing agent batch transaction
• add merge-aware cell resolution, rollback/idempotency/byte-preservation receipts, and candidate version 3.1.0
• clarify first-party identity and the optional real-Hancom visual oracle boundary

Why

S-079 consolidates accumulated form-fill paths around one typed, revision-bound transaction without creating a second package executor.

Validation

• 1514 passed, 44 skipped, 5 xfailed
• Ruff E9/F, compileall, mypy, and pyright pass
• wheel/sdist build and Twine checks pass
• isolated wheel install, metadata, package data, and CLI smoke pass

Release publication remains separately owner-gated.