stream: group bool fields at tail of serverStream to eliminate false sharing with mu

[gidotencate]

Reorder `serverStream` fields so both bool fields are grouped at the tail of the struct, and move `mu` to a separate cache line from the tail bools.

Why

`serverStream` had 2 `bool` fields placed between aligned fields, each forcing alignment padding before the next field:

Field	Padding after
`recvFirstMsg`	7 B (before `int`, align 8)
`serverHeaderBinlogged`	3 B (before `sync.Mutex`, align 4)

Total: 8 B wasted. Grouping both bools at the tail eliminates the interior padding, reducing `serverStream` from 256 B → 248 B, saving 8 B per server-side RPC stream unconditionally.

Additionally, `serverHeaderBinlogged` is written unsynchronised in `SendHeader` and `Send` (it is documented as not needing synchronization). In the tail-grouped layout, the bools land at offset 240+ — which was the same cache line as `mu` (offset 232, cache line 3: 192–255). Concurrent writes to `serverHeaderBinlogged` invalidate the cache line before every `mu.Lock()`, adding 5–21× latency overhead under concurrent load.

Moving `mu` to immediately before `trInfo` (which it guards) places it at offset 184 (cache line 2: 128–191). The tail bools remain at offset 240+ (cache line 3), so the false-sharing is eliminated. No size change from this step — `serverStream` stays at 248 B.

See benchmarks in #9349.

Benchmark — `mu.Lock()` latency with stressor goroutines writing `serverHeaderBinlogged` concurrently:

Stressors	Original layout	After tail-group only	After moving mu to CL2
0	3.9 ns	3.9 ns	3.7 ns
1	20.6 ns (5.2×)	20.1 ns (5.1×)	3.8 ns (~1×)
2	46.1 ns (11.7×)	45.8 ns (11.7×)	3.7 ns (~1×)
4	81.9 ns (20.8×)	79.9 ns (20.4×)	3.7 ns (~1×)

Code clarity

Both bools are unguarded. The bool group uses a single `// Not guarded by mu` section, preserving the existing per-field comments. `mu` is placed immediately before `trInfo` (which it guards) with a comment explaining the cache-line placement.

Closes #9349

RELEASE NOTES:

• core: reorder `serverStream` fields to eliminate alignment padding (saving 8 bytes per server-side RPC stream) and cache-line false sharing between `serverHeaderBinlogged` and `mu` (up to 21× `mu.Lock()` speedup under concurrent load).

[gidotencate]

@easwars @mbissa Could one of you assign this PR (and linked issue #9349) to @gidotencate? I don't have write access to assign myself.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.50%. Comparing base (9d1988d) to head (b91c821).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9361      +/-   ##
==========================================
+ Coverage   87.45%   87.50%   +0.05%     
==========================================
  Files         425      425              
  Lines       30297    30303       +6     
==========================================
+ Hits        26496    26517      +21     
+ Misses       3801     3786      -15     

Files with missing lines	Coverage Δ
stream.go	88.43% <ø> (+0.23%)	⬆️

... and 25 files with indirect coverage changes

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[eshitachandwani]

LGTM , adding @mbissa as a second reviewer.

[gidotencate]

Done, removed in the latest commit.

[mbissa]

/gemini review

[gemini-code-assist]

Code Review

This pull request reorganizes the fields of the serverStream struct in stream.go to group boolean fields at the end, eliminating alignment padding and reducing the struct's memory footprint. There are no review comments, and we have no additional feedback to provide.

[mbissa]

LGTM