[WASM W3] Make repository tests host-portable

[cpunion]

Tracks xgo-dev#2152 and is stacked on #252.

This is the portable repository-test layer of W3. It replaces native-host assumptions with explicit WebAssembly contracts so the following acceptance layer can execute the applicable test/** inventory instead of accumulating broad skips. It contains no compiler or runtime production changes.

Scope

• Model supported and unsupported os, os/exec, signal, network, runtime/debug, heap-dump, and profiling behavior explicitly on WebAssembly hosts.
• Keep applicable behavior under test while moving genuinely native-only caller, finalizer, linkname, and goroutine-lifecycle cases behind narrow build constraints.
• Compare encoded WebAssembly function PCs correctly and retain runtime function-information assertions.
• Bound recursive type and RSA fixture work for fixed WebAssembly stacks without weakening the semantic assertions.
• Generate deterministic binary fixtures portably instead of relying on host object formats or pre-existing native artifacts, including tested decode and materialization failures.
• Preserve official GoJS browser-host contracts for HTTP, os/user, and testing.Chdir.
• Accept the post-Go 1.26 panic/recover semantics already selected by the Go 1.27 toolchain.

Validation

Current head 42689cb40d27, based on #252 5ddb2fd8c6ed (10 commits, 77 files, +1,595/-426).

• git range-diff reports all 9 rebased commits equivalent to the previously audited range; the tenth commit only adds binary-fixture failure coverage. git diff --check passes.
• Every changed file is under test/; the range adds no t.Skip, TODO/FIXME marker, diagnostic probe, dependency override, or production implementation.
• The final-head six-profile WebAssembly standard-library gate passes for J32-GoJS (wasm32 with the Go-compatible JavaScript host), J32-Emscripten (wasm32 with Emscripten), J64-Emscripten (wasm64 with Emscripten Memory64), W32-WASI (wasm32 with WASI Preview 1), and the official GoJS/GoWASI references.
• The preceding equivalent-head focused WASM runtime and public test-command jobs pass; the final delta changes only the independently tested fixture helper.
• The final-head Ubuntu instrumented coverage job passes. Codecov project and patch checks pass at 93.87% and 95.0%; the fixture materialization helper itself is 100% covered, with only its defensive testing.TB.Fatal caller guard unexecuted.
• The final-head format gate passes.
• A size benchmark is intentionally omitted because every changed file is test-only, so this range cannot change release modules.

Review boundary

This PR only normalizes repository tests and fixtures. The complete sharded test/**/GOROOT inventory, per-profile sentinels, accounting artifacts, and real-browser GoJS callback execution remain in the following W3 acceptance PR.

[codecov]

Codecov Report

❌ Patch coverage is 95.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 93.87%. Comparing base (5ddb2fd) to head (42689cb).

Files with missing lines	Patch %	Lines
test/internal/binaryfixture/fixture.go	95.00%	1 Missing ⚠️

Additional details and impacted files

@@                            Coverage Diff                             @@
##           codex/wasm-aggregate-lowering-w2-20260912     #255   +/-   ##
==========================================================================
  Coverage                                      93.87%   93.87%           
==========================================================================
  Files                                            158      159    +1     
  Lines                                          30658    30678   +20     
==========================================================================
+ Hits                                           28780    28799   +19     
- Misses                                          1877     1878    +1     
  Partials                                           1        1           

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[cpunion]

Superseded by the consolidated W3 contribution stacked on xgo-dev#2580. All 10 commits are preserved with exact range-diff equivalence in the combined 32-commit acceptance range.