fix(cluster): serve metadata reads at or above the client's own writes

[numinnex]

A client that commits a metadata write and then re-homes its session onto a backup can be served the pre-write state. auth.rs already documents the gap: the session epoch is the register's commit op, and on a backup that forwarded the proposal the local applied commit still lags it. A backup applies committed ops only as its commit walk advances, and nothing tied a read to the op the client's own write committed.

Metadata reads now gate on the connection's committed watermark. One applied-frontier counter per process advances after every metadata apply and is shared with every shard, which for the first time gives a shard without consensus the applied position it had no way to observe. The watermark comes from the commit field replies already carry, seeded at bind from the session epoch. The fast path is a single atomic load with no awaits, so the shared-nothing read path is unchanged; a lagging node parks briefly, then fails the read retryable rather than answering stale.

Over HTTP this closes the forwarded-register case, where a healthy backup forwards the register so the bound epoch can exceed the local frontier. It does not close the case where forwarding is active: the follower relays the write, its handler never runs, so the node that later serves the read holds no session and no watermark. Closing that needs the serving primary's commit op to travel back to the reading node, for instance a response header beside the view the forward middleware already relays. That is additive but touches every control-plane write response, so it is left out here and documented at the gate.

The BDD delete-then-get steps now assert "not the stream we deleted" instead of "nothing at this id". The server hands a deleted stream's numeric id straight to the next create, so once scenarios share a server the old assertion cannot hold, and removing the polling loop without this would have left the spec flaky for an unrelated reason.

[codecov]

Codecov Report

❌ Patch coverage is 92.45283% with 36 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.05%. Comparing base (328b289) to head (9d2e8ee).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
core/simulator/src/lib.rs	90.10%	15 Missing and 3 partials ⚠️
core/server/src/dispatch.rs	78.57%	11 Missing and 1 partial ⚠️
core/server/src/http/reads.rs	93.75%	3 Missing and 2 partials ⚠️
core/server/src/http/submit.rs	80.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #4024      +/-   ##
============================================
+ Coverage     85.00%   85.05%   +0.04%     
  Complexity     1402     1402              
============================================
  Files          1225     1225              
  Lines        180283   180752     +469     
  Branches     146587   147056     +469     
============================================
+ Hits         153248   153735     +487     
+ Misses        22993    22952      -41     
- Partials       4042     4065      +23     

Components	Coverage Δ
Rust Core	85.95% <90.90%> (+0.05%)	⬆️
Java SDK	67.29% <ø> (ø)
C# SDK	75.44% <ø> (+0.07%)	⬆️
Python SDK	90.06% <ø> (ø)
PHP SDK	85.65% <ø> (ø)
Node SDK	96.24% <ø> (ø)
Go SDK	69.35% <ø> (+0.03%)	⬆️

Files with missing lines	Coverage Δ
core/metadata/src/impls/metadata.rs	88.19% <100.00%> (+0.10%)	⬆️
core/server/src/bootstrap.rs	80.77% <100.00%> (+0.28%)	⬆️
core/server/src/http.rs	92.96% <ø> (ø)
core/server/src/http/error.rs	87.31% <100.00%> (+0.34%)	⬆️
core/server/src/http/extractor.rs	85.71% <100.00%> (+0.46%)	⬆️
core/server/src/http/reply.rs	90.03% <100.00%> (ø)
core/server/src/http/session.rs	96.11% <100.00%> (+0.43%)	⬆️
core/server/src/http/state.rs	88.54% <100.00%> (+0.12%)	⬆️
core/server/src/session_manager.rs	91.41% <100.00%> (+1.03%)	⬆️
core/simulator/src/client.rs	96.53% <100.00%> (+0.18%)	⬆️
... and 5 more

... and 32 files with indirect coverage changes

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