feat: PLA-1061 accept immutable case admission order

[jeffatgametime]

The following code makes an impact.

Purpose of Change

Allow a hosted orchestration caller to supply an immutable test-case admission order without changing selected coverage or result identity. This is the generic library primitive for duration-informed scheduling, not the history selector.

Ticket Link

PLA-1061

Change Overview

• Add optional RunOptions.CaseOrder, keyed by package and containing a complete case-name permutation.
• Validate the full hint before admission; unknown, missing or duplicate names trigger whole-hint fallback with a package diagnostic.
• Preserve canonical result indices/names, sequential child steps, hooks, cleanup and existing shared/exclusive executor admission.
• No result-store access, scheduling clock, new dependency or static package priority.

Stack dependency: #37 (itself follows #36). This PR targets hosted-case-concurrency while #37 is open. Do not merge into that feature branch. Retarget to master only after prerequisites merge, revalidate the diff, CI and default-branch review requirements.

Affected Functionality

Hosted RunWithContext/RunPackageWithOptions admission order only. Missing hints preserve canonical behavior. Native Go test ordering remains unchanged. Hints are caller-owned immutable snapshots; the library does not mutate them.

Testing

• Independent go test -race ./... -count=3 -timeout=90s: pass, including shared executor lifecycle/exclusivity tests.
• go vet ./..., git diff --check: pass.
• Matching Go 1.21.13 + golangci-lint 1.54.2: clean.
• Tests prove valid permutation changes execution order while retaining canonical results/child identities, and wrong cardinality, same-length duplicates, unknown names and absent hints execute every case once via deterministic fallback. Invalid hints emit a diagnostic.
• Fresh review caught incorrect test expectations for nested names; primary corrected expectations to existing parent/name identities before the passing verification runs.

Observability

Invalid scheduling hints emit a package diagnostic. Existing QueueDur and admitted lifecycle Dur remain separate. Runtime duration history selection, comparable coverage and queue-delay measurements belong to the api-tests consumer integration; no speedup claimed here.

Rollout

Merge prerequisites #36 and #37 first, retarget and reverify on master, then normal checks/required teammate approval. No tag or deployment by this PR-management task. Root owns immutable api-tests dependency adoption, recorded history snapshot, Temporal compatibility and staged runtime validation. Remove CaseOrder or pass no hint for canonical-order fallback.