chore(api): add @Beta annotation + apply to NodeDefinition SPI (H2)

[DemchaAV]

Summary

Wires up Track H2 from the v1.6.5→1.7 readiness taskboard. The G3 stability policy (#88) referenced @Beta as the planned Extension SPI / Experimental marker; this PR ships the annotation, applies it to the first concrete Extension SPI seam, and lifts the "pending" hedges in the policy doc.

Changes

New annotation — com.demcha.compose.document.api.Beta

Sibling to the existing @Internal marker. Same shape — @Documented, @Retention(RUNTIME), @Target({TYPE, METHOD, FIELD, CONSTRUCTOR, PACKAGE}) — so architecture guard tests can read both reflectively. Lives next to @Internal so IDE autocomplete surfaces both markers.

The annotation's docstring documents two usages:

• Extension SPI (common case) — the seam is intentionally public so users can implement or call it; shape may evolve in a minor release with a one-minor deprecation window where possible.
• Experimental — a brand-new public type shipping in its first minor release before the contract has stabilised; may change or disappear in any minor release without a deprecation window.

The distinction lives in the annotated element's own Javadoc, not in the annotation itself.

First application — NodeDefinition is now @Beta

com.demcha.compose.document.layout.NodeDefinition is the canonical Extension SPI seam — the interface library users implement to register custom node types. Its Javadoc now explicitly explains the seam ("carved out of the otherwise @Internal document.layout package") and links the API stability policy § 1 Extension SPI row.

Additional Extension SPI surfaces (render-handler interfaces, fragment-payload interfaces designed for extension) will gain the marker incrementally as their contract solidifies — tracked as follow-up.

Tests — BetaAnnotationDocumentationTest

Pins the annotation's contract in the same shape InternalAnnotationDocumentationTest pins @Internal:

Assertion	What it locks
RUNTIME retention	Guard tests can read @Beta reflectively (matches @Internal).
@Documented	Javadoc surfaces the marker on annotated types.
Target set	TYPE / METHOD / FIELD / CONSTRUCTOR / PACKAGE — same as @Internal.
Source-Javadoc invariants	Mentions both "Extension SPI" and "Experimental", links docs/api-stability.md, directs users to the issue tracker.
Same-package check	Beta.class.getPackage() == Internal.class.getPackage() — IDE autocomplete invariant.

Documentation updates

• docs/api-stability.md — @Beta reference cells in § 1 are no longer hedged as "pending"; the associated quote block lists both markers side-by-side with the guard tests that pin them. NodeDefinition now linked from the Extension SPI row.
• docs/architecture/package-map.md — new intro paragraph documents the stability-marker convention; the document.layout row calls out NodeDefinition as the current @Beta seam.

Verification

$ ./mvnw -B -ntp test -pl . \
    -Dtest='BetaAnnotationDocumentationTest,InternalAnnotationDocumentationTest,InternalAnnotationCoverageTest,CanonicalSurfaceGuardTest,DocumentationCoverageTest,PublicApiSinceTagCoverageTest,PublicApiNoEngineLeakTest'
BUILD SUCCESS (25s)

$ ./mvnw -B -ntp test -pl .
Tests run: 1029 (+5 from BetaAnnotationDocumentationTest), Failures: 0

CHANGELOG entry added to v1.6.6 — Planned under new ### Public API and existing ### Documentation subsections.

Test plan

• BetaAnnotationDocumentationTest 5/5 green
• Existing InternalAnnotationCoverageTest / InternalAnnotationDocumentationTest unchanged and still green
• G3 stability-policy doc no longer references a "pending" annotation
• Package-map intro and document.layout row carry the marker convention
• CI green on PR (Guards / JDK 17/21/25 / Examples Smoke / binary-compat)
• Reviewer skim — Beta.java (60 LOC) and the policy-doc revisions are where the human eye pays off; the test mirrors the existing Internal test 1:1
• (Out of scope) follow-up: incrementally apply @Beta to the rest of the Extension SPI surface (render-handler interfaces, fragment-payload interfaces) as each seam's contract solidifies