chore(api): add class-level @since to public entry points + coverage guard (H1)

[DemchaAV]

Summary

Wires up Track H1 from the v1.6.5→1.7 readiness taskboard. The G3 stability policy (#88) pins what @since is supposed to mean; this PR makes it true on the canonical entry-point surface.

26 public types in the canonical user-reached packages now carry class-level @since 1.0.0 Javadoc tags so callers see the introduction version at IDE quick-doc / generated Javadoc time without trawling CHANGELOG history. A new architecture guard fails the build if a future public top-level type lands in these packages without a class-level @since.

What changes

@since 1.0.0 added (class-level Javadoc)

Package	Types tagged
com.demcha.compose	GraphCompose
com.demcha.compose.document.api	DocumentSession, DocumentPageSize, PageBackgroundFill
com.demcha.compose.document.dsl	DocumentDsl, RichText, Transformable + 19 builders (AbstractFlowBuilder, BarcodeBuilder, CanvasLayerBuilder, DividerBuilder, EllipseBuilder, ImageBuilder, LayerStackBuilder, LineBuilder, ListBuilder, ModuleBuilder, PageBreakBuilder, PageFlowBuilder, ParagraphBuilder, RowBuilder, SectionBuilder, ShapeBuilder, ShapeContainerBuilder, SpacerBuilder, TableBuilder)

Total: 26 files.

Baseline choice — why @since 1.0.0

These are foundational types that have shipped since the first GraphCompose release and predate the CHANGELOG history that could pin a more specific version. The pragmatic baseline communicates "this is in the API from the beginning"; new public types added in subsequent PRs use the upcoming release version (e.g. @since 1.6.6).

New guard — PublicApiSinceTagCoverageTest

Source-scans three entry-point roots:

• src/main/java/com/demcha/compose/GraphCompose.java
• src/main/java/com/demcha/compose/document/api/
• src/main/java/com/demcha/compose/document/dsl/

Fails the build if a .java file declares a public top-level type (class / interface / record / enum / annotation) and @since does not appear in the source above the first such declaration. package-info.java and files in internal/ sub-packages are excluded by convention — InternalAnnotationCoverageTest already covers @Internal packages and the same naming rule applies to internal/ sub-packages.

Why this scope, not a full sweep

• Method-level backfill for the ~380 public methods in these packages is intentionally out of scope. Method @since is now policy for all new public methods (the graphcompose-senior-review skill enforces it in Lane 5's Javadoc checklist) but retrofitting the existing surface is its own task — and one that benefits from per-method git archaeology that an automated sed pass can't safely do.
• Other public packages (document.node, document.style, the template surfaces) are not scanned. The taskboard scopes H1 to the entry-point surface — GraphCompose factory, the document.api session / builder seam, and the document.dsl authoring builders. A broader sweep follows the same pattern when it lands.

Verification

$ ./mvnw -B -ntp test -pl . -Dtest=PublicApiSinceTagCoverageTest
Tests run: 1, Failures: 0, Errors: 0, Skipped: 0

Mentally reverting any single @since line goes red — confirmed by sed-removing one tag in RowBuilder.java and re-running the guard (then restoring it).

$ ./mvnw -B -ntp test -pl .
Tests run: 1024 (+1 from PublicApiSinceTagCoverageTest), Failures: 0, Errors: 0

Full suite green confirms the bulk Javadoc edits didn't break Javadoc parsing or unrelated tests.

CHANGELOG entry added to v1.6.6 — Planned under the existing ### Build subsection.

Test plan

• New guard green against current state
• Full suite green (1024 / 0 / 0)
• All 19 builders + 7 non-builder entry points carry class-level @since 1.0.0
• CI green on PR
• Reviewer skim — most edits are mechanical (the sed-generated @since 1.0.0 line); the guard test itself (~95 LOC) is where the human review pays off
• (Out of scope) follow-up: method-level @since backfill — separate PR, separate cadence; the guard does not enforce method-level today