feat: PLA-1062 report skipped test coverage explicitly

[jeffatgametime]

The following code makes an impact.

Purpose of Change

Represent skipped prerequisite coverage explicitly without turning unexecuted assertions into passes or losing cleanup/failure evidence.

Ticket Link

PLA-1062

Change Overview

• Add optional testy.Skipf / testy.Skipped capabilities without expanding TestingT.
• Record skipped leaves, incomplete containers and skip reasons; add skipped-aware statistics while preserving the existing statistics signature.
• Preserve failure precedence through defers, cleanup, native/hosted body panics, and nested tests.
• Show skipped counts and incomplete coverage in stored summaries and bundled result templates.

Affected Functionality

Generic testy lifecycle, result aggregation, in-memory summaries and HTML rendering. This is the library foundation only; API-tests prerequisite policy and consumer reporting are a separate dependent change in progress. The library can merge independently; consumers must explicitly handle incomplete results before enabling skips. No case concurrency or Domain Model web-404 change is included.

Testing

• go test -count=1 -race ./... and go vet ./... passed with Go 1.25.1.
• Existing repository lint configuration passed using Go 1.21.13 and golangci-lint 1.54.2 (matching CI's version families).
• Independent subprocess probes verified native body/cleanup panics cannot be masked by later cleanup skips; parent skips preserve incomplete coverage.
• Independent template execution verified skipped reason/counts and warning presentation.
• Go 1.21 binaries compile locally but this macOS rejects execution with dyld: missing LC_UUID; Linux CI remains authoritative for that runtime.

Observability

Results retain SkipReason even when a real failure takes precedence. Skipped tests remain in total coverage but never passed counts. Containers expose incomplete, and skipped counts are available through SumTestStatsWithSkipped and Summary.Skipped. Cleanup failures are not suppressed by the generic library.

Rollout

Merge and consume an immutable library revision through the normal dependency update. No service deployment belongs to this library PR. API-tests adoption will separately update persisted results/UI/Slack/check policies and verify hosted behavior. PLA-1062 remains In Progress until that companion verification is complete. Roll back adoption by reverting the consumer dependency/policy change through its normal PR path.