Add DCB exists/count read-option overloads and pin write-path limitations

[johanhaleby]

What

Follow-ups from the deep review of the DCB write path. The two correctness findings, the unconditional-append skew and the read-watermark overshoot, already landed in the write-path PR. This one cleans up the smaller items.

• DcbEventStore.exists and count now have DcbReadOptions overloads, so you can ask "does anything match in this window" the same way read already scopes by position. The Spring Mongo store answers them with a direct exists or count query. The in-memory store uses the read-based default.
• DcbReadOptions.between now names its parameters afterSequencePosition and upToSequencePosition to match the rest of the type. The exclusive and inclusive semantics stay in the javadoc.
• Two tests pin behavior that was only documented before: the position-window overloads, and the MatchAll concurrency limitation. The MatchAll test shows that a MatchAll condition does not detect a concurrent tag-scoped append, so a later change cannot quietly weaken it.
• ADR 0021 now spells out that markers are not reclaimed automatically, and how an operator can prune them safely during quiescence.

Two items I left alone, with reasons

The review also flagged the Kotlin DcbExecuteOptions<*> cast and the decider DSL not taking DcbExecuteOptions. Both rest on the premise that the stream side does it differently. It does not.

• The stream Kotlin extensions use the exact same @Suppress("UNCHECKED_CAST") over ExecuteOptions<*> with an as ExecuteOptions<E> cast. The DCB code mirrors it on purpose. A variance-based fix is reasonable, but it belongs on both at once, not on DCB alone where it would drift from the established pattern.
• The stream decider extensions do not take ExecuteOptions either. Adding options only to the DCB decider DSL would create the asymmetry the finding wanted to remove, not fix it.

I would rather keep DCB consistent with the stream DSL than half-apply either change. Happy to do both as a separate cross-cutting pass if you want the variance cleanup.

Tests

All green: DcbApiTest 15, InMemoryEventStoreDcbTest 20, SpringMongoEventStoreDcbTest 16, including the new position-window and MatchAll-limitation tests.

[github-actions]

Qodana Community for JVM

74 new problems were found

Inspection name	Severity	Problems
Kotlin Maven Plugin misconfigured	🔶 Warning	27
Constant values	🔶 Warning	25
Nullability and data flow problems	🔶 Warning	14
'Optional' used as field or parameter type	🔶 Warning	2
Pointless boolean expression	🔶 Warning	2
'equals()' called on classes which don't override it	🔶 Warning	1
Result of method call ignored	🔶 Warning	1
Function or property has platform type	◽️ Notice	2

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[johanhaleby]

Landed via the consolidated #234.