stream: group bool fields at tail of clientStream to reduce allocator size class 288B→256B

[gidotencate]

Reorder clientStream fields so all 7 bool fields are grouped at the tail of the struct. Add (s).TestClientStreamSize to guard against regressions.

Why

clientStream had 7 bool fields scattered among pointer- and int-sized fields. Each bool was followed by 3–7 bytes of alignment padding before the next 4- or 8-byte-aligned field, totalling ~40 bytes of wasted padding:

Field	Padding after
sentLast + receivedFirstMsg (together)	6 B
serverHeaderBinlogged	3 B (before sync.Mutex, align 4)
firstAttempt	7 B (before int, align 8)
finished	7 B (before pointer, align 8)
committed	7 B (before func(), align 8)
nameResolutionDelay	7 B tail padding

Result: 280 B struct → 288 B allocator size class.

Grouping all bools at the tail packs them into 7 B + 1 B tail pad = 8 B, reducing the struct to 248 B → 256 B allocator size class: 32 B saved per RPC stream unconditionally, ~10.8% less GC pressure at scale (benchmarks in #9280).

Code clarity

Lock discipline is preserved explicitly: the bool group is split into // Not guarded by mu and // Guarded by mu sections with per-field comments matching the original. A comment in the struct explains the layout rationale and points to #9280. (s).TestClientStreamSize gives a clear error message directing future contributors to add new bool fields in the grouped section.

Closes #9280

RELEASE NOTES:

• core: reorder clientStream bool fields to reduce struct size from the 288-byte to the 256-byte allocator size class, saving 32 bytes per RPC stream.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: gidotencate / name: Gido ten Cate (9ed2533)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.14%. Comparing base (6f508fa) to head (f441de4).
⚠️ Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9281      +/-   ##
==========================================
+ Coverage   83.05%   83.14%   +0.09%     
==========================================
  Files         422      422              
  Lines       35040    35044       +4     
==========================================
+ Hits        29101    29139      +38     
+ Misses       4427     4403      -24     
+ Partials     1512     1502      -10     

Files with missing lines	Coverage Δ
stream.go	82.46% <ø> (ø)

... and 32 files with indirect coverage changes

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