Implement reassignment for arrays declared using const generics

[ironcev]

Description

This PR implements reassignments for arrays declared using const generic arguments:

fn const_generic_array<const N: u64>() -> [u8; N] {
    let mut res = [0u8; N];
    res[0] = 1;          // <<<<--- This now works.
    res
}

During development and testing of this PR, several other issues are found. Those issues will be addressed in separate PS, and are marked as TODOs in code, linking to their appropriate GitHub issues.

• Closes Reassignment to const generic array emits "Type is not indexable" error #7600.

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
  • If my change requires substantial documentation changes, I have requested support from the DevRel team
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.

[cursor]

PR Summary

Medium Risk
Touches semantic-analysis projection typing and compile-time array bounds checking, which can affect correctness of indexing/reassignment and error reporting across many programs. Changes are localized but in a core type-checking path and add/adjust multiple e2e snapshots.

Overview
Enables array element projection/type resolution to work for arrays whose length is not a compile-time literal (e.g., const-generic [T; N]), while still emitting ArrayOutOfBounds only when both the index and the array length can be evaluated as literals.

Updates diagnostics/help text for NotIndexable to mention generic arrays/references, renames the negative test program array_bad_index to array_invalid_index_type, and expands array_oob_const_index to cover global/local const indices and const-generic lengths (with updated snapshot expectations).

Adds a new should-pass e2e program const_generics_array_in_reassignments to validate reassignment/indexing on const-generic arrays and nested arrays.

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[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[codspeed-hq]

Merging this PR will degrade performance by 16.01%

❌ 1 regressed benchmark
✅ 24 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
❌	tokens_for_program	534.4 µs	636.3 µs	-16.01%

---

_{Comparing ironcev/const-generic-array-not-an-indexable-type (217ecd1) with master (e12e611)}

[ironcev]

👍