[rush reporter][R7] Integrate Heft child reporting and problem matchers

[TheLarkInn]

Parent: #5858

Blocked by: #5976, #5978

Goal

Carry the semantic reporter stream through Heft child processes while retaining compatibility with older Heft versions and opaque external tools.

Scope

• Negotiate the child descriptor/NDJSON protocol and forward correlated Heft envelopes.
• Propagate reporter/log-level/color/width context to child processes.
• Retain ordered raw stdout/stderr fallback for older or incompatible Heft versions.
• Register and run problem matchers against the uncollated source stream.
• Preserve parent/child ordering, source sequence, privacy classification, and record/chunk limits.

Acceptance criteria

• New Rush + new Heft uses negotiated structured events.
• New Rush + old Heft remains readable through ordered raw fallback.
• Unknown optional events degrade safely; unsupported required features produce an actionable diagnostic.
• Problem matchers recover diagnostics without mutating raw output.
• Cross-version and malformed-stream tests cover fallback behavior.

DAG

Unblocks R9.

Part of #5858.