Enable opt-in R8 JNI name obfuscation for CoreCLR and NativeAOT - #12575

## Summary

Related to #12535

This is layer 1 of 6 in the replacement stack for PR #12575. It extracts the self-contained R8/JNI name-mapping primitives and their focused unit tests so later layers can add build integration and runtime-specific behavior independently.

This layer intentionally changes no build behavior. Nothing invokes these new helpers yet.

## What this adds

- `R8Mapping`
  - Parses R8/ProGuard mapping files, including class, field, method, constructor, overload, line-range, inline-frame, and class-merging entries.
  - Reports the actual source mapping path and line number when file parsing fails.
  - Buffers positional records by residual range and obfuscated name as R8 retrace stacks, indexing only the final residual frame as callable so same-class inline-only methods are not rewritten.
  - Preserves every original class candidate when R8 merges several classes into one residual class.
  - Uses rewrite-manifest class entries to disambiguate reverse class and member mappings, failing closed unless exactly one original class remains allowed.
  - Fails closed when reverse field or name-only method lookup remains ambiguous, while allowing a manifest filter to select one candidate safely.
  - Records the owning class together with every successful forward member lookup, allowing the resulting manifest to restrict its own reverse mapping safely.
  - Tracks accessed entries atomically across concurrent lookups and exposes stable snapshots to manifest consumers.
  - Reports a removed declaring class even when reachability input starts with only a required field or method entry, without emitting duplicate class conflicts.
  - Provides compatibility and reachability conflict detection used by later stack layers.
  - Produces and consumes compact, sorted manifest entries with deterministic `\n` newlines.
- `JniDescriptorText`
  - Validates JNI field and method descriptors, including non-empty slash-separated object type names.
  - Rejects malformed object descriptors containing Java source-name dots, array markers inside class names, or empty path segments.
  - Rewrites embedded object type names while preserving primitive and unrelated types.
  - Converts validated JNI descriptor tokens into Java source type names without redundantly rescanning tokens parsed from a method descriptor.
  - Preserves nested binary-name `$` separators when converting descriptors to R8 mapping keys.
- `LdstrRewriter`
  - Rewrites JNI-sensitive string constants using an `IJniNameMapping` implementation.
  - Handles encoded method and field IDs, constructor descriptors, `RegisterNatives` entries, multiline registration blocks, and exact JNI class-name strings.
  - Leaves unrelated or unresolved strings unchanged.

## Test coverage

Adds focused tests for:

- R8 mapping syntax, source-aware parse errors, positional inline stacks, qualified and same-class inline frames, residual method preservation, merged-class reverse lookup, manifest disambiguation, zero/multiple-candidate failure, concurrent access tracking and snapshot semantics, member-only owning-class tracking, forward and reverse lookups, overloads, constructors, nested classes, deterministic manifest output, and conflict detection.
- JNI method/field descriptor validation, malformed object descriptors, object and array rewriting, Java type conversion, nested `$` descriptor-to-mapping-key conversion, and malformed standalone token handling.
- JNI-related `ldstr` forms, including encoded member IDs, constructors, registration blocks, class names, unchanged inputs, and a forward/reverse field round trip restricted by the forward access manifest.

Validation commands:

```sh
./dotnet-local.sh test src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/Xamarin.Android.Build.Tests.csproj -v minimal --filter 'FullyQualifiedName~R8MappingTests|FullyQualifiedName~JniDescriptorTextTests|FullyQualifiedName~LdstrRewriterTests'
./dotnet-local.sh build src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/Xamarin.Android.Build.Tests.csproj --no-restore -v minimal
```

Result: **82 passed, 0 failed, 0 skipped**; test project build succeeded with **0 warnings and 0 errors**.

## Intentionally out of scope

The following remain for later layers in the replacement stack:

- Build integration and MSBuild tasks/targets.
- Metadata assembly rebuilding.
- Typemap `FieldRVA` handling.
- CoreCLR integration.
- NativeAOT integration.
- User-facing documentation.

The initial implementation and tests in this layer were reconstructed from the verified PR #12575 source snapshot at `ac2b30593ff5afa0659fe35d3de515239c7540fe`, followed by descriptor-validation, ambiguity-safety, diagnostics, concurrency, merged-class, inline-frame, and owning-class access-tracking hardening from review. PR #12575 and its branch remain unchanged.

## Summary

This is layer 2 of 6 in the replacement stack for PR #12575. It adds the internal PE and metadata reconstruction substrate required by later JNI name-rewriting layers, without enabling any build behavior.

- Rebuilds managed PE images while preserving existing metadata tokens and supported table row ordering.
- Re-emits method bodies, embedded resources, debug-directory data, native resources, and `FieldRVA` mapped data, including ranges from different PE sections.
- Preserves overlapping/aliased `FieldRVA` backing data by relocating rewritten fields instead of modifying shared storage.
- Validates malformed IL, resource offsets/directories, and strong-name signature directories through controlled rewrite errors.
- Emits event/property method semantics explicitly in `HasSemantics` coded-index order.
- Rejects unsupported field-backed `ImplMap` rows instead of silently dropping metadata.
- Adds low-level metadata encoding, raw-column, IL operand-scanning, and rewrite-plan primitives.
- Covers longer and shorter UTF-8 `FieldRVA` replacements, aliased mapped fields, event/property accessor preservation, per-use `#US` splitting, and strong-name signature preservation directly through `AssemblyRebuilder`.
- Uses the executing runtime's core assembly version in generated fixtures so rebuilt images load across target frameworks.

## Scope

This layer intentionally does **not** add JNI-specific planning or attribute/`ldstr` orchestration, the assembly-rewriter facade, the `RewriteJniNamesForR8` MSBuild task, typemap rewrite behavior, CoreCLR/NativeAOT target integration, or documentation. Those surfaces belong to later stack layers.

## Validation

- `./dotnet-local.sh build src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/Xamarin.Android.Build.Tests.csproj -c Debug -v:minimal --no-restore`
- `./dotnet-local.sh test bin/TestDebug/net10.0/Xamarin.Android.Build.Tests.dll --filter 'FullyQualifiedName~NativeResourceSectionCopierTests|FullyQualifiedName~AssemblyRebuilderTests'`
- Direct layer-2 result: 18 passed, 0 failed.
- Cumulative layer-3 `RewrittenAssemblyLoadsAndRunsInTheRuntime` result: 1 passed, 0 failed.

Related to #12535

Depends on #12628