docs: align CLAUDE.md with dual Java 8/11+ target

The build now ships as a Java 8 artifact (JDK 8 is the source of truth,
CI builds/tests/releases on it) while still compiling on JDK 11+. Update
the project guidance to describe the dual-target profiles and the Java 8
language/API floor, and list the newer-than-8 features that are off-limits
(var, List.of/copyOf, String.repeat, ProcessHandle, Thread.onSpinWait,
the Java 9 Provider(String,String,String) ctor, enhanced switch, text
blocks, instanceof pattern vars).

Author: bluestreak01

CLAUDE.md

@@ -7,9 +7,30 @@ Guidance for Claude Code when working with the QuestDB Java client
 
 The standalone client repo for QuestDB ingestion (legacy ILP and QWP). It is
 a Git submodule of the `questdb` repository and ships as the
-`io.questdb:questdb-client` Maven artifact. The module targets Java 11, so
-only legacy Java features are available here — no enhanced switch, no
-multiline strings, no pattern variables in `instanceof`.
+`io.questdb:questdb-client` Maven artifact.
+
+**Dual JDK target — always aim at both.** The client ships as a Java 8
+artifact (JDK 8 is the source of truth: CI builds, tests, and releases on
+JDK 8) but must also compile cleanly on JDK 11+ (CI runs a JDK 25
+compile/javadoc smoke job). The right Maven profile (`java8` /
+`java11+`) auto-activates from the running JDK; a `src/main/java8` vs
+`src/main/java11` source root supplies the per-JDK shims. Practically this
+means **Java 8 is the language/API floor** — every change must build on
+both, so write to Java 8 and never introduce an API or syntax that only a
+newer JDK has.
+
+Disallowed here (newer than Java 8), among others:
+- `var` local-variable type inference (10), enhanced `switch` (14),
+  multiline text blocks (15), pattern variables in `instanceof` (16)
+- `List.of` / `List.copyOf` / `Map.of` and friends (9/10),
+  `String.repeat` / `String.strip` / `String.isBlank` (11)
+- `ProcessHandle` (9), `Thread.onSpinWait` (9),
+  the `Provider(String, String, String)` ctor (9 — use the Java 8
+  `Provider(String, double, String)` overload)
+
+When in doubt, check the API's `@since` and keep it `<= 1.8`. JDK
+9+-only behaviour belongs behind the `src/main/java11` shim with a Java 8
+counterpart in `src/main/java8`.
 
 ## Git & PR Conventions
 
@@ -35,13 +56,18 @@ multiline strings, no pattern variables in `instanceof`.
 
 ## Build
 
-Standard Maven build. Targets Java 11:
+Standard Maven build. Dual-target (Java 8 floor, Java 11+ also supported);
+the `java8` / `java11+` profile auto-activates from the JDK running Maven:
 
 ```bash
 mvn clean install -DskipTests   # build + install to local Maven cache
 mvn -pl core test                # run client unit tests
 ```
 
+Build on a JDK 8 to validate the shipping artifact (the source-of-truth
+target); also confirm it compiles on a modern JDK (11+) before merging, the
+same two fronts CI guards.
+
 The parent `questdb` repo's `local-client` profile pulls this module as a
 sub-module so server changes can build against unpublished client code; if
 you change client code, install it (or pass `-P local-client` in the parent)