Add two standalone modules: Stream Gatherers (JEP 485) and Scoped Values vs ThreadLocal (JEP 506)

gatherers/: windowFixed, windowSliding, fold, scan, mapConcurrent, a custom
Gatherer (dedupeConsecutive, takeUntil), and a Collector-vs-Gatherer comparison.
Verified: windowSliding emits one short window on a too-short stream rather than
shrinking to empty like windowFixed; mapConcurrent waits for in-flight siblings
to finish (~901ms) rather than cancelling them when one mapper throws.

scoped-values/: ScopedValue API and rebinding, why plain threads (virtual or
platform) do not inherit a binding while a StructuredTaskScope subtask does,
and the memory cost vs InheritableThreadLocal measured two ways -- a noisy
heap-delta first pass, then a precise jcmd GC.class_histogram object census
(5 shared Carrier objects vs ~700,000 copied ThreadLocalMap/Entry objects for
100,000 threads).

Companion code for the ankurm.com posts "Java Stream Gatherers (JEP 485)" and
"Scoped Values vs ThreadLocal in Java 25: Migration Guide with Virtual Threads".

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01FtpJvZfg4nvLvtzgJTDWpB

docs/output/99-inheritance.txt

@@ -0,0 +1,8 @@
+$ java --enable-preview InheritanceRequiresStructuredTaskScope      (JDK 25)
+Thread.ofVirtual().start(...) from inside the bound block: UNBOUND
+CHECK ok   : a plain virtual thread started from inside where().run() does NOT see the binding -- REQUEST_ID.get() threw NoSuchElementException
+new Thread(...) from inside the bound block: UNBOUND
+CHECK ok   : a plain platform Thread doesn't see the binding either -- this isn't a virtual-thread-specific rule
+StructuredTaskScope.fork(...) from inside the bound block: req-42
+CHECK ok   : a subtask forked from a StructuredTaskScope opened while REQUEST_ID is bound DOES see "req-42"
+exit=0