fix: propagate Tag and Context fields through Logger.With

[costela]

zapsentry.Tag and zapsentry.Context were only honored when passed directly to the log call. Fields attached earlier via zap's Logger.With were silently dropped, because core.with() did not carry them into the child core and Write() only scanned the call-site fields.

Store tags and the context on the core in with(), and have Write() read them from the already-computed clone. Also added regression tests for both fields.

Fixes #56

[costela]

FYI, the benchmarks look slightly better in some cases. The ones that look worse are actually the feature working as it should 🙈

pkg: github.com/TheZeroSlave/zapsentry
cpu: AMD Ryzen AI 9 HX 370 w/ Radeon 890M
                           │   old.txt   │               new.txt               │
                           │   sec/op    │   sec/op     vs base                │
Write_PlainFields            5.123µ ± 4%   4.903µ ± 4%   -4.29% (p=0.006 n=10)
Write_CallSiteTag            5.401µ ± 4%   5.561µ ± 4%   +2.97% (p=0.029 n=10)
Write_ErrorField             5.995µ ± 3%   5.821µ ± 5%        ~ (p=0.143 n=10)
Write_DerivedLoggerWithTag   5.593µ ± 3%   5.487µ ± 1%   -1.90% (p=0.011 n=10)
With_Tag                     475.4n ± 2%   867.4n ± 3%  +82.43% (p=0.000 n=10)
With_PlainField              905.7n ± 2%   940.3n ± 5%   +3.82% (p=0.000 n=10)
geomean                      2.714µ        2.987µ       +10.07%

                           │   old.txt    │               new.txt                │
                           │     B/op     │     B/op      vs base                │
Write_PlainFields            4.422Ki ± 0%   4.406Ki ± 0%   -0.35% (p=0.000 n=10)
Write_CallSiteTag            4.445Ki ± 0%   4.727Ki ± 0%   +6.33% (p=0.000 n=10)
Write_ErrorField             4.523Ki ± 0%   4.508Ki ± 0%   -0.35% (p=0.000 n=10)
Write_DerivedLoggerWithTag   4.352Ki ± 0%   4.336Ki ± 0%   -0.36% (p=0.000 n=10)
With_Tag                       416.0 ± 0%     752.0 ± 0%  +80.77% (p=0.000 n=10)
With_PlainField                976.0 ± 0%    1008.0 ± 0%   +3.28% (p=0.000 n=10)
geomean                      2.305Ki        2.579Ki       +11.91%

                           │  old.txt   │               new.txt                │
                           │ allocs/op  │ allocs/op   vs base                  │
Write_PlainFields            35.00 ± 0%   34.00 ± 0%   -2.86% (p=0.000 n=10)
Write_CallSiteTag            35.00 ± 0%   35.00 ± 0%        ~ (p=1.000 n=10) ¹
Write_ErrorField             38.00 ± 0%   37.00 ± 0%   -2.63% (p=0.000 n=10)
Write_DerivedLoggerWithTag   34.00 ± 0%   33.00 ± 0%   -2.94% (p=0.000 n=10)
With_Tag                     7.000 ± 0%   8.000 ± 0%  +14.29% (p=0.000 n=10)
With_PlainField              9.000 ± 0%   9.000 ± 0%        ~ (p=1.000 n=10) ¹
geomean                      21.53        21.71        +0.80%
¹ all samples are equal