OPENNLP-1888: Document annotation container with typed offset-anchored layers

[krickert]

Adds the document annotation container discussed on OPENNLP-1888: an immutable Document over the original text, typed LayerKey identities, span-anchored Annotation values, DocumentAnnotator with declared requires/provides, and a DocumentAnalyzer whose pipeline ordering is validated at build time. Four adapters over the existing single-task interfaces (sentence detector, tokenizer, POS tagger, name finder) plus lemmatizer and stemmer layer adapters come with it; the container itself never learns about specific layers.

Contract behavior, each pinned by a test asserting the exact message where one is thrown: spans are structurally mandatory and validated against the text length; key equality is the (id, type) pair; layers preserve insertion order and are never sorted; layers are immutable once added and detached from caller input; providing an already-present layer is rejected; reading an absent layer returns an empty immutable list. 33 tests, including a full pipeline example and a contract suite.

Follow-ups planned as commits on this PR, from the review discussion on the ticket: namespaced identifiers for the standard keys; per-key positional versus document scope for whole-document facts; the invariants above transcribed into the specification text; and the documented convention for gold versus predicted layers. Opening as a draft until those land.

The acceptance criterion suggested in that discussion, that a new layer can be added without touching the container package, is already observable: the feature branches on the ai-pipestream fork (glossary, PII, coreference, dependencies, relations, money/quantity/temporal, geo, embeddings) each add their layers with no container edits.