Complete shadow reporter parity coverage

[Sean Larkin (TheLarkInn)]

Part of #5976

Stack

#5985 copilot/reporter-r1a-package-wiring
  -> #5986 copilot/reporter-r1b-bootstrap-generation
    -> #5987 copilot/reporter-r2a-experiment-config
      -> #5989 copilot/reporter-r2b-frontend-host-controls
        -> #5988 copilot/reporter-r3a-session-sink
          -> #5991 copilot/reporter-r3b-shadow-events
            -> this PR #5992 copilot/reporter-r3c-shadow-parity

Complete merge order: #5985 -> #5986 -> #5987 -> #5989 -> #5988 -> #5991 -> #5992.

R6 #5993 is a parallel fork from #5989 and is not part of this child chain.

Summary

• add exhaustive shadow parity coverage for event ordering, correlation, trusted source identity, and privacy-safe telemetry projection
• prove derived exit status matches success, warning-only success, operation failure, logical cancellation, configuration errors, and internal errors
• verify project x phase operation identities remain distinct and stable across repeated watch-style iterations
• prove stdout, stderr, and legacy StreamCollator rendering remain byte-identical while shadow reporting emits no raw output
• cover no-sink/legacy frontend behavior by verifying scoped producers remain absent unless the optional engine sink is supplied
• fix the shadow exit adapter so callers can preserve the canonical logical-cancellation outcome without changing authoritative process exit behavior

Independent stack review

• Expose scoped RushSession reporter producers #5988 is narrowly scoped to the optional frontend-to-engine sink and trusted Rush/action/plugin producer facades; no identity spoofing, privacy leak, output change, or compatibility defect was found
• Emit shadow Rush lifecycle events #5991 keeps legacy rendering and process exit authoritative, does not route operation chunks, and correctly correlates emitted operation diagnostics with legacy errors
• the missing cancellation derivation and exhaustive parity matrix are addressed here rather than expanding either parent slice
• telemetry assertions intentionally verify the Harden reporter telemetry privacy projection #5990 privacy boundary without duplicating its reporter-package implementation

Validation

• focused Rush-lib reporter parity tests: 41 passed
• rush test --only @rushstack/rush-reporter --only @microsoft/rush-lib --only @microsoft/rush --only @rushstack/rush-sdk (8 operations passed)
• rush build --to @microsoft/rush --to @rushstack/rush-sdk (52 operations passed)
• rush check
• rush change --verify --no-fetch

No-output guarantee

No sink is created unless explicitly supplied. The shadow operation adapter emits lifecycle/status/diagnostic events only, never externalOutput; focused tests compare stdout, stderr, and the complete legacy StreamCollator rendering byte-for-byte with and without shadow reporting.

Non-goals

• no reporter selection, CLI flags, repository defaults, or automatic activation
• no StreamCollator replacement or R5 raw operation/output routing
• no duplication of Harden reporter telemetry privacy projection #5990 telemetry-subscriber privacy hardening
• no changes to legacy terminal/logger APIs or authoritative process exit behavior
• no child-to-parent merges while stack ancestors remain open

[Sean Larkin (TheLarkInn)]

Combined deep review of current head 22089a0957.

No critical, high, or medium actionable findings. No useful low follow-ons.

[Sean Larkin (TheLarkInn)]

Restacked onto exact #5991 head 2f9ac7d90e365143c0b33da2e8dd9782fda99e35. New #5992 head: 7a751fc03f0a089f0279e8fb50000733a02fc47e. The PR is one R3C commit, four files, CLEAN, mergeable, and auto-merge remains disabled. The R3C parity additions were preserved while overlapping sharded and watch behavior remains owned by #5991. Combined stack validation passed.

[Sean Larkin (TheLarkInn)]

Final combined review of 7a751fc03f found no critical, high, medium, or useful low issues. The restack preserves parity, cancellation, ordering, privacy, exit authority, and byte-identical visible output. This head is review-clean.