fix(blocks): Post no longer silently drops items on a full bounded channel

[jeremydmiller]

Problem

Block<T>.Post wrote to a channel bounded at 10,000 using a non-blocking TryWrite, and silently dropped every item once the channel filled — the only trace was a Debug.WriteLine. Any caller that posts faster than the reader drains loses data with no error surfaced.

This is the root cause of wolverine GH-3287: a handler cascading 20,000 messages to a buffered local queue only delivered ~10,000. It also silently affects Marten's ProjectionUpdateBatch (a projection producing >10k storage operations in one batch would drop the overflow).

Fix

• Post back-pressures instead of dropping. On a full bounded channel it now block-waits for capacity via WriteAsync (BoundedChannelFullMode.Wait). Safe for the common case where the poster is not the block's own reader.
• Configurable capacity + Unbounded option. New Block(int parallelCount, int boundedCapacity, action) ctor; pass Block<T>.Unbounded for blocks that must never block or drop. The parameterless/(parallelCount, action) ctors keep the DefaultBoundedCapacity (10,000) — no behavior change for existing callers except that overflow now blocks rather than drops.
• RetryBlock uses an unbounded internal block. executeAsync re-enqueues failed items onto its own block from within its processing action; against a full bounded channel with back pressure that self-re-post would deadlock, so retries must never block on write.

Downstream (Wolverine)

Wolverine's buffered local queue will opt into Block<Envelope>.Unbounded (a local queue is routinely a handler's own cascade target, so bounded back pressure could deadlock the single reader). That change ships in the paired Wolverine PR once 2.19.1 is published.

Tests

• InMemoryQueueTests.synchronous_Post_does_not_drop_items_when_the_bounded_channel_fills — holds the reader closed until the channel saturates and the producer is blocked in Post, proving the overflow item is back-pressured, not dropped; then drains all 25,000.
• InMemoryQueueTests.unbounded_block_never_blocks_or_drops_on_Post.
• Full CoreTests suite green (453 passed), including existing RetryBlock tests.
• Verified end-to-end against Wolverine via a local 2.19.1 feed: the GH-3287 reproduction (20k cascaded messages) delivers all 20,000 at parallelism 5 and 1.

Bumps to 2.19.1.

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