Add SbSmallMap for the profiler texture cache

[Dikluwe]

Summary

• add a private SbSmallMap for trivial key/value pairs, using SbList's four inline entries before one contiguous spill allocation
• define the type, invalidation, allocation-failure, and noexcept equality contracts and cover them with direct and differential tests
• replace the profiler's six-entry texture std::map with SbSmallMap
• verify that repeated visualizations reuse each cached texture while all six texture categories remain distinct

Measurements

Instrumentation of the real profiler path observed the fixed expected workload: six insertions, 18 lookups, maximum cardinality six.

A local Release benchmark reproducing six insertions, 18 lookups, one iteration, and cleanup measured these medians over five complete runs:

Container	Lifecycle	Steady lookup	Allocations per lifecycle
SbSmallMap	73.87 ns	2.49 ns	1
std::map	188.43 ns	2.73 ns	6

The benchmark and instrumentation are intentionally kept outside this PR and are used only as local engineering evidence.

Stack

Depends on #756. Until that PR is merged, GitHub also shows its two SbList commits in this diff. This PR intentionally introduces the private container together with its first measured consumer instead of landing unused infrastructure.

Testing

• complete Release build
• CTest: 8/8 passed
• direct spill-allocation failure and recovery coverage
• ASan run completed without an error attributable to this change
• UBSan repeated the pre-existing SoPath/SoFullPath downcast finding outside this diff