feat: add explicit tuple expression scope references

[Seddryck]

Adds ^$n, ^^$n, and deeper tuple references to select the input of an exact expression scope. Nested invocations preserve their enclosing inputs; pipeline stages and grouping do not add scopes. Missing scopes and invalid tuple access return null without fallback, while unqualified $n behavior is preserved.

Uses native TupleProjectionSyntax roots from Expressif.Syntax 0.37.0. Binding reads RootDepth directly, preserving authored source text, root children, and spans. Binding, evaluation, serialization, CLI inspection, documentation, and conformance coverage are included. Invalid scoped coercion selectors are rejected rather than losing their scope.

Validation:

• Solution build: zero warnings/errors; StyleCop analyzers clean.
• Full .NET 10 library suite: 4,897 passed, one skipped.
• Native syntax and scope checks: 58 passed on each of .NET 8, 9, and 10.
• CLI suite: all 238 tests passed.

close #975

[coderabbitai]

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Run ID: b34fbdd7-29fe-49c9-b17e-86cb9a8efb81

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[codecov]

Codecov Report

❌ Patch coverage is 93.82716% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.20%. Comparing base (0493eeb) to head (e409332).

Files with missing lines	Patch %	Lines
Expressif/Bindings/ExpressifBinder.cs	86.95%	1 Missing and 2 partials ⚠️
Expressif/Syntax/ExpressionParser.cs	92.00%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #979      +/-   ##
==========================================
+ Coverage   86.11%   86.20%   +0.08%     
==========================================
  Files         290      290              
  Lines        8945     9017      +72     
  Branches     1655     1671      +16     
==========================================
+ Hits         7703     7773      +70     
  Misses        792      792              
- Partials      450      452       +2     

Flag	Coverage Δ
expressif-cli-net10.0	33.22% <30.86%> (-0.06%)	⬇️
expressif-net10.0	84.87% <91.25%> (+0.09%)	⬆️
expressif-net8.0	84.87% <91.25%> (+0.09%)	⬆️
expressif-net9.0	84.87% <91.25%> (+0.09%)	⬆️
expressif-serialization-net10.0	4.39% <0.00%> (-0.05%)	⬇️
expressif-serialization-net8.0	4.39% <0.00%> (-0.05%)	⬇️
expressif-serialization-net9.0	4.39% <0.00%> (-0.05%)	⬇️

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[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud