Name the callee zero test cannot execute in test failures

[guygrigsby]

Makes zero test failures name the callee the runner could not execute instead of appending only the test label. Addresses the diagnostic confusion in #428; the underlying limitation (the test interpreters execute only std.testing helpers and pure zerolang functions) is unchanged.

Before:

zero test unknown function: my test

After, for a callee that does not resolve:

zero test unknown function 'myHelper': my test

After, for a real stdlib function the runner does not execute, matching the existing unsupported-construct wording:

zero graph test runner does not implement 'std.mem.span' yet; tests can call std.testing helpers and pure zerolang functions; use live smoke tests for other std calls: std call coverage

Changes:

• Thread the callee name and call node into test_eval_function and pgt_eval_function so the existing null-function guard is the single place the message is produced, in both interpreters. Failures now anchor to the unresolved call's location instead of the test function's.
• std.-prefixed callees get the not-implemented framing; other unresolved names keep unknown function '<name>'.
• Rewrite the skill-data/testing.md paragraph that read this failure as a malformed test label and advised deleting the test; regenerated embedded_skills.inc is included.
• Add a command contract pinning the message and failure.sourceFile for a package test reaching std.mem.*, and adjust two file line budgets in scripts/compiler-metrics.mts.

Verified locally on darwin-arm64: make -C native/zero-c, pnpm run command-contracts:local, pnpm run conformance:local, pnpm run native:test:local, pnpm run native:sanitize, pnpm run workspace:checks all pass. The :sandbox variants were not run locally (they require VERCEL_OIDC_TOKEN); CI covers those paths.

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