fix(server-ng): eliminate unnecessary backpressure under concurrent clients

[numinnex]

Cancel-safe commits: on_ack now peeks → reads → revalidates → applies
atomically instead of popping before its awaits; a dropped driver (e.g.
canceled HTTP handler) no longer strands committed entries and panics the
shard. HTTP register/logout submit via detached tasks.

Serialized checkpoints: forced checkpoints in on_replicate are gated
single-flight (concurrent drain()s raced the shared wal.tmp rewrite),
and a failed checkpoint no longer drops the prepare — that left a phantom
op that permanently gapped the commit frontier until the pipeline wedged.

Backpressure absorbed, not surfaced: submits arriving while an op is
mid-commit are queued (register, with its reply subscriber riding the
request queue) or simply pipelined (logout / requests, matching the wire
path) instead of bouncing transient NotCaughtUp — which one-shot clients
saw as hard login/logout failures.

[codecov]

Codecov Report

❌ Patch coverage is 83.76384% with 176 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.29%. Comparing base (499d8aa) to head (bec3a4f).

Files with missing lines	Patch %	Lines
core/metadata/src/impls/metadata.rs	86.90%	78 Missing and 4 partials ⚠️
core/server-ng/src/dispatch.rs	77.88%	46 Missing ⚠️
core/server-ng/src/http/submit.rs	0.00%	14 Missing ⚠️
core/server-ng/src/http/state.rs	0.00%	9 Missing ⚠️
core/journal/src/prepare_journal.rs	91.17%	5 Missing and 1 partial ⚠️
core/server-ng/src/bootstrap.rs	0.00%	5 Missing ⚠️
core/configs/src/server_ng_config/displays.rs	0.00%	4 Missing ⚠️
core/configs/src/server_ng_config/metadata.rs	93.33%	4 Missing ⚠️
core/configs/src/server_ng_config/validators.rs	0.00%	2 Missing and 1 partial ⚠️
core/metadata/src/impls/recovery.rs	88.88%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             master    #3692       +/-   ##
=============================================
- Coverage     74.06%   60.29%   -13.77%     
  Complexity      937      937               
=============================================
  Files          1301     1301               
  Lines        147464   136412    -11052     
  Branches     123017   112038    -10979     
=============================================
- Hits         109224    82256    -26968     
- Misses        34768    50601    +15833     
- Partials       3472     3555       +83     

Components	Coverage Δ
Rust Core	57.46% <83.76%> (-16.83%)	⬇️
Java SDK	62.44% <ø> (ø)
C# SDK	71.04% <ø> (-1.14%)	⬇️
Python SDK	92.17% <ø> (ø)
PHP SDK	84.29% <ø> (ø)
Node SDK	92.40% <ø> (-0.10%)	⬇️
Go SDK	42.87% <ø> (ø)

Files with missing lines	Coverage Δ
core/configs/src/server_ng_config/defaults.rs	100.00% <100.00%> (ø)
core/configs/src/server_ng_config/server_ng.rs	47.05% <ø> (ø)
core/consensus/src/impls.rs	75.06% <100.00%> (+0.80%)	⬆️
core/consensus/src/plane_helpers.rs	94.02% <100.00%> (+0.10%)	⬆️
core/shard/src/lib.rs	54.14% <100.00%> (+0.02%)	⬆️
core/configs/src/server_ng_config/validators.rs	72.37% <0.00%> (-0.86%)	⬇️
core/metadata/src/impls/recovery.rs	79.71% <88.88%> (+2.23%)	⬆️
core/configs/src/server_ng_config/displays.rs	0.00% <0.00%> (ø)
core/configs/src/server_ng_config/metadata.rs	93.33% <93.33%> (ø)
core/server-ng/src/bootstrap.rs	11.15% <0.00%> (-0.03%)	⬇️
... and 5 more

... and 262 files with indirect coverage changes

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