[Based on #2098] runtime,test: complete Go 1.26 standard-library coverage

[cpunion]

Depends on #2098

Problem

The Go 1.26 upgrade added packages and exported APIs that were not all represented by test/std. The coverage job only checked packages that already had a test directory, so a missing public package was invisible. Executable-only tests also did not prove that standard-library code initialized and ran correctly from c-shared and c-archive C consumers.

Fix

• require a test/std package for every public package reported by Go 1.26.5 (176/176);
• keep the existing exported-identifier checker unchanged;
• add functional tests for missing packages and newly exposed APIs, including HPKE, ML-KEM helpers, runtime/cgo.Handle, runtime/debug, plugin failure behavior, and testing/synctest;
• add the minimal Go 1.26 runtime bridges exposed by those tests without duplicating runtime,cl: add Go-compatible sampled memory profiling #2027's memory-profile work;
• teach llgo test -c to produce runnable c-shared and c-archive test libraries;
• run every test/std package as an executable on Linux and macOS, and through real C shared/static consumers on Ubuntu.

The tests call APIs and assert behavior; exported-symbol completeness remains a separate compile-time invariant.

This PR has an independent diff of +1156/-25 across 36 files.

Validation

• macOS arm64 / Go 1.26.5: changed runtime/debug tests pass with real LLGo.
• Ubuntu arm64 / Go 1.26.5 / LLVM 19.1.7: runtime/debug passes as an executable, c-shared, and c-archive in an 8 GiB, 2 CPU, no-swap container.
• Public package coverage remains 176/176.
• Focused internal/build tests pass.

Follow-up to #2088 and #2108.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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