chore(api): graduate the SVG surface from beta to stable

[DemchaAV]

Why

The SVG surface has carried @Beta since 1.8.0 with an explicit exit condition — "while the surface hardens against real-world exporter output". That condition is met: the stroke/colour/unit hardening landed through 1.8.x (#180, #184–#186, #193), clip-path support in 1.9.0, and the opacity family plus the approximation warnings in #596 — while the API shape itself has not moved since 1.9.0, four minors of real use (the 2.0 line only relocated the files). The stability policy in docs/api-stability.md says an Experimental surface graduates "typically by the next minor release", and its own registry of current @Beta carriers already lists only NodeDefinition and the PPTX backend — the code disagreed with the policy doc. The marker was also internally inconsistent: inlineSvgIcon / RichText.svgIcon and the emoji pipeline were built on @Beta SvgIcon without carrying the annotation themselves.

What changed

• Eight annotation sites dropped: SvgIcon, SvgPath (class-level), PathBuilder.svg(svgPath), both AbstractFlowBuilder.addSvgIcon(...) overloads and ShapeContainerBuilder.path(w, h, svgPath) (these three used the fully-qualified @com.demcha.compose.document.api.Beta form — a plain-text @Beta grep misses them), and DocumentPaint.LinearAxis / RadialCircle. The paints graduate together with the reader that emits them: SvgIcon.Layer exposes DocumentPaint publicly, so a beta paint under a stable icon would re-create the same inconsistency this PR closes.
• The four "Beta:" Javadoc paragraphs removed (SvgIcon, SvgPath, document.svg package-info, and the "may shift while that reader hardens" caveats on both paint records) — the surrounding docs already state the supported subset, the out-of-scope list, and each normalization contract, which is the stable promise. One stale test comment ("would DoS the @beta reader") updated alongside.
• CHANGELOG ### Public API entry under v2.2.1 — Planned calls the transition out explicitly, as the policy requires, and names the two remaining @Beta carriers (NodeDefinition Extension SPI, PPTX backend), which now match docs/api-stability.md exactly — no doc edit needed there.
• No behaviour, signature, or binary change — annotation and Javadoc only. BetaAnnotationDocumentationTest pins the annotation's shape, not its carriers, so no test changes.

Verification

Full reactor gate ./mvnw -B -ntp clean verify -pl :graph-compose-core,…,:graph-compose-coverage -am → BUILD SUCCESS (exit 0, unpiped), and the Javadoc gate ./mvnw -B -ntp javadoc:javadoc → BUILD SUCCESS — the removed {@link Beta} references leave no dangling links. japicmp stays green: annotation removal is not a binary-compatibility event, and the plugin carries no @Beta exclusion that could have been load-bearing.

Lane: canonical (document.svg, document.dsl, document.style) — annotation/docs only.

Follow-up to #596 (the hardening pass this graduation was gated on).