Complete the minimal 1.0 API additions

[hokoo]

Linked issue

Closes #2
Closes #3
Closes #4

Intent

Complete the three approved minimal API additions after the immutable 0.7.0
baseline: initialize from an existing WP_Post, expose slug accessors, and expose
menu-order accessors. T12 API freeze and any further surface changes remain out
of scope.

Behavior

• Before: initialization accepted only a weakly typed integer ID/new-post input;
  slug and menu order had no symmetric interface/trait accessors.
• After: initialization accepts exactly int|WP_Post|null; supplied post objects
  retain identity and use the existing validation/meta/hook lifecycle without an
  insert or lookup. Typed chainable slug and menu-order accessors mutate the
  corresponding WP_Post fields in memory and persist through savePost().
• Migration or recovery: trait consumers receive both accessor pairs
  automatically. Manual interface implementations/overrides must implement the
  documented signatures. Numeric strings, floats, and booleans previously
  accepted by weak int coercion must be converted explicitly. No stored-data
  migration is required.
• Out of scope: slug sanitization/permalink policy, menu-order range/reordering
  policy, autosave, T12 visibility/interface freeze, and RC publication.

SemVer impact

• none — no package release required by this change alone
• PATCH — backward-compatible bug or security fix
• MINOR — feature/deprecation, or a pre-1.0 compatibility change
• MAJOR — incompatible change starting with 1.0

Rationale: this is the approved pre-1.0 public API expansion and includes an
explicitly documented initialization-input compatibility change.

Compatibility and security

• Public/protected API, signatures, visibility, return values, or chainability
• Exception types or machine-readable error context
• WordPress hook names, arguments, ordering, timing, or defaults
• Existing metadata or JSON persistence and migration
• Composer dependencies or production archive contents
• PHP/WordPress support bounds or current-version compatibility
• Security, permissions, external input, secrets, or diagnostic output
• None of these surfaces is affected

Risk and compatibility details: wppaLoadPostException remains the mismatch
contract and preserves ID/model context; invalid non-union inputs now fail with
TypeError before side effects. Existing hook/meta/JSON behavior is shared and
regression-tested. Slug normalization/uniqueness remains owned by Core and is
observed after reload. Any signed integer menu order is accepted without a new
query/reordering promise. PHP 7.4 syntax and WordPress 6.0 remain supported.
The package boundary and secret/artifact scan are clean.

Verification evidence

Check	Result	Evidence or reason
make check	Pass	59 tests, 354 assertions; Composer strict validation and PHP lint pass
make coverage	Pass	180/180 source lines and 43/43 methods (100%)
make test.package-install	Pass	Clean production archive/install; all public symbols autoload
make smoke	Pass	Existing localdev create/save/reload/meta/param/publish lifecycle
make test.integration.minimum	Pass	WP 6.0 / PHP 7.4.33; minimum-20260910T233033Z-47568-5574; 16 lifecycle groups; zero fixtures; empty diagnostics
make test.integration.latest	Pass	WP 7.1 / PHP 8.4.25; latest-20260910T233211Z-49057-3120; 16 lifecycle groups; zero fixtures; empty diagnostics
git diff --check	Pass	master...dca2940 clean
Other targeted check	Pass	Direct PHP 7.4 unit 59/354; independent E3 contract/regression QA PASS with no findings or missing AC/DoD

Documentation

• README.md
• VERSIONING.md
• docs/LOCAL-DEVELOPMENT.md
• docs/TESTING.md
• docs/RELEASING.md
• CONTRIBUTING.md or repository templates
• Public code/API documentation
• Other documentation named below
• No documentation change is required

Documentation rationale: README covers consumer usage and save/reload semantics;
VERSIONING records the compatibility surface and manual-implementer migration;
TESTING records the expanded real-WordPress lifecycle; docs/plan-1.0.md records
every task transition and exact evidence.

Changelog

• Added a user-oriented entry under Unreleased in CHANGELOG.md
• No changelog entry is valid for this pull request

Changelog entry or concrete no-changelog rationale: Unreleased now records
object initialization, typed slug/menu-order accessors, and upgrade notes for
constructor inputs and manual interface implementations.

Final review

• The branch is focused and excludes unrelated changes.
• Tests assert behavior and failure paths, not only coverage percentage.
• No credentials, private .env values, generated dependencies, local
  WordPress files, coverage reports, or integration artifacts are included.
• Published tags and release artifacts are unchanged.
• Remaining risks, follow-ups, and checks not run are stated above.
• Required CI checks are green, or the PR remains a draft pending them.

Final synchronized run
34543321838
passed all five required checks. T9–T11 issues stay in review until this PR
merges.