Optimize Color/Brush parsing and string conversion, reduce allocs

[h3xds1nz]

Currently blocked by #9364, hence draft.

Description

Optimizes parsing of colors and brushes from string and also conversion of colors to string.
It currently cherry-picks the ref-struct tokenizer from #9364, but it is directly dependent due to several changes over those previously done.

ParseContextColor (normal)

Method	Mean [ns]	Error [ns]	StdDev [ns]	Code Size [B]	Gen0	Allocated [B]
Original	528.9 ns	10.28 ns	13.01 ns	3,138 B	0.0763	1288 B
PR__EDIT	331.3 ns	2.51 ns	2.22 ns	1,517 B	0.0114	192 B

ParseContextColor (edge case)

Method	Mean [ns]	Error [ns]	StdDev [ns]	Code Size [B]	Gen0	Allocated [B]
Original	687.2 ns	6.62 ns	5.87 ns	2,971 B	0.1040	1752 B
PR__EDIT	384.9 ns	0.90 ns	0.84 ns	1,517 B	0.0114	192 B

ToString with Round-trip format

Method	Mean [ns]	Error [ns]	StdDev [ns]	Gen0	Code Size [B]	Allocated [B]
Original	425.0 ns	1.64 ns	1.37 ns	0.0181	3,864 B	304 B
PR__EDIT	321.7 ns	0.89 ns	0.79 ns	0.0048	833 B	80 B

ToString with ColorContext

Method	Mean [ns]	Error [ns]	StdDev [ns]	Gen0	Code Size [B]	Allocated [B]
Original	601.2 ns	4.31 ns	3.60 ns	0.0668	4,029 B	1120 B
PR__EDIT	419.1 ns	1.29 ns	1.15 ns	0.0138	1,506 B	232 B

Source data

// Benchmark 1
"ContextColor file://C:/Windows/system32/spool/drivers/color/RSWOP.icm 1.0, 0.0, 0.5, 1.0, 1.0"
// Benchmark 2
"ContextColor file://C:/Windows/system32/spool/drivers/color/RSWOP.icm 1.0,            1.0, 0.5,       0.0,       1.0"
// Benchmark 3+4
private static MILColorF scRgbColor = new() { a = 0.7667f, b = 0.2254f, g = 1.0f, r = 0.8458488f };
// Benchmark 4
uriString = "file://C:/Windows/system32/spool/drivers/color/RSWOP.icm",

Customer Impact

Improved performance, decreased allocations.

Regression

No.

Testing

Local build, assert tests.

Risk

Low to medium; will follow-up with unit tests once we're in that timeframe and past WPF tests.

Microsoft Reviewers: Open in CodeFlow

[h3xds1nz]

Resolved merge conflicts, validated with #10633 and #10632 tests.

[codecov]

Codecov Report

❌ Patch coverage is 68.98551% with 107 lines in your changes missing coverage. Please review.
✅ Project coverage is 11.19259%. Comparing base (3cc9d57) to head (75a04d6).
⚠️ Report is 272 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #10225         +/-   ##
===================================================
+ Coverage   11.17950%   11.19259%   +0.01308%     
===================================================
  Files           3314        3315          +1     
  Lines         665182      665226         +44     
  Branches       74668       74684         +16     
===================================================
+ Hits           74364       74456         +92     
+ Misses        589525      589474         -51     
- Partials        1293        1296          +3     

Flag	Coverage Δ
Debug	11.19259% <68.98551%> (+0.01308%)	⬆️

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