feat: multi-turn conversation support (0.4.0)

anonymizeTurn(text, session) threads a serializable AnonymizerSession
({mapping, legend, history}) so one entity keeps one id across a whole chat
(applyKnown reuse + usedIds + de-collision merge). Adds conversation()
in-memory wrapper and an optional LlmProvider.anonymizeInConversation(text, ctx)
for rich cross-turn context; providers without it fall back to the batch path.

openAICompatibleProvider gains includeMappingInContext (default false — only
send real values to a trusted anonymizer endpoint) + historyMaxTurns (default
10). Verified live vs Gemma 4: Nora stays PER_1 across 4 turns (name/email/AVS/
IBAN), Yanis = PER_2; 41 tests, 98% coverage.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -4,6 +4,24 @@ All notable changes to this project are documented here. The format is based on
 [Keep a Changelog](https://keepachangelog.com/en/1.1.0/), and this project adheres to
 [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [0.4.0] - Unreleased
+
+### Added
+
+- **Multi-turn conversation support.** New `Anonymizer.anonymizeTurn(text, session?) → { anon, mapping,
+legend, session }` keeps one stable id per entity across a whole chat: it seeds each turn with a running,
+  serializable `AnonymizerSession` (`{ mapping, legend, history }`), reuses known values via `applyKnown`,
+  tells the model which ids are taken, and de-collides new ones. Persist the returned `session` and pass it
+  back next turn.
+- `Anonymizer.conversation(initial?)` — a stateful in-memory wrapper (`anonymize`, `deanonymize`,
+  `session()`) over `anonymizeTurn`.
+- Optional `LlmProvider.anonymizeInConversation(text, ctx)` — providers can use prior context (anonymized
+  `history`, `legend`, `usedIds`, and optionally `mapping`) for better cross-turn coreference/attribution.
+  `openAICompatibleProvider` implements it; providers that don't fall back to the batch path automatically.
+- `openAICompatibleProvider` options: `includeMappingInContext` (**default false** — only send real values
+  to a _trusted_ anonymizer endpoint) and `historyMaxTurns` via `AnonymizerConfig` (default 10).
+- Exported the `AnonymizerSession` type.
+
 ## [0.3.1] - Unreleased
 
 ### Changed