Add typed self-counting WASM sections

[jtschuster]

Summary

This is the second PR in the WASM object-writer stack and builds on #131770.

• Replaces shared functionality in WasmSection and WebCilSection for emitting a SectionWriter's data to SectionDataEmitter.
• Change WebCilSection's base class from WasmSection to SectionDataEmitter to clarify that it's not an actual custom Wasm section.
• Add typed vector sections for imports, functions, globals, exports, and elements. Adding entries / data to these sections should be done through their respective methods that take an entry object and the corresponding SectionWriter. This allows them to accurately track the count of entries in the section.
• Type and Code are externally counted since entries can be added to from the base ObjectWriter.
• Remove the corresponding manual counters and count-prefix rewriting from the writer.

The emitted module structure and existing writer behavior are preserved while making section counts follow directly from successfully written rows.

Stack

1. Extract WASM section model #131770 — Extract WASM section model
2. Add typed self-counting WASM sections #131771 — Add typed self-counting WASM sections (this PR)
3. Introduce WebCIL object writer type #131772 — Introduce WebCIL object writer type
4. Move shared WASM logic to base writer #131778 — Move shared WASM logic to base writer

Note

This PR description was generated with GitHub Copilot assistance.

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[Copilot]

Pull request overview

This PR refactors WASM section emission in the CoreCLR object writer to use typed, self-counting “vector” sections (imports/functions/globals/exports/elements), moving per-row encoding into section-specific WriteEntry implementations and eliminating manual counters / count-prefix patching in the writer.

Changes:

• Introduces WasmVectorSection / WasmSection<TEntry> abstractions that prepend the vector length automatically and increment entry counts only after successful entry encoding.
• Updates WasmObjectWriter to use Add* APIs that delegate row encoding/counting to typed section implementations, and finalizes Type/Code counts via an externally-counted section type.
• Moves WasmExportKind to a shared location (WasmNative.cs) for reuse by section code.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs	Switches writer logic to typed sections (Add* methods) and removes manual count handling; finalizes external counts for Type/Code.
src/coreclr/tools/Common/Compiler/ObjectWriter/WasmNative.cs	Adds shared WasmExportKind enum for export section encoding.
src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSections.cs	Adds typed GetSection<T> helpers to retrieve sections safely by name/index.
src/coreclr/tools/Common/Compiler/ObjectWriter/Wasm/WasmSection.cs	Replaces PrependCount with a generalized content-prefix mechanism and adds self-counting vector section implementations for key WASM sections.

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