[Pepps233]

Summary

Adds a private, runtime-neutral JSONL trace foundation that implements the existing callback-based telemetry contract.

One trace can now represent a complete Diffex process invocation across session bindings and live harness activations without importing either runtime or requiring AgentHarness completion.

Runtime wiring, transcript materialization, analysis, and evolution changes remain out of scope for this PR.

Reasoning

Immutable producer identity belongs in the trace header, while session and harness state can change during one process invocation and must be represented as ordered records.

Format-specific cursors preserve truthful durability boundaries without coupling the trace package to coding-agent or AgentHarness internals.

A streaming validator makes incomplete and corrupted traces diagnostically useful by retaining valid records while reporting structural failures.

Exclusive per-invocation files, serialized writes, and passive post-creation failures keep tracing isolated from application behavior.

Changes

• Add the private @earendil-works/diffex-harness-trace package with only the telemetry package as a runtime dependency.
• Keep immutable producer identity in the header, including Diffex version, source revision, runtime kind and version, trace schema version, and telemetry schema versions.
• Add explicit session_attach records with stable binding IDs, session identity, path, format metadata, and attachment reasons.
• Add harness_activate records for initial activation and live harness switching.
• Replace numeric session ranges with session_segment records using coding-agent v3 entry cursors or AgentHarness durable sequence bounds.
• Validate coding-agent entry ordinals, entry IDs, and SHA-256 prefix or content digests without importing coding-agent code.
• Preserve strict single-record encoding, exclusive file creation, serialized writes, nested spans, events, span settlement, health counters, flush(), idempotent close(), missing-footer crash detection, and passive writer degradation.
• Add a streaming whole-file reader that detects sequence gaps, duplicate or misplaced headers and footers, invalid span parentage, duplicate settlement, records after closure, partial final lines, missing footers, and incompatible trace schemas while retaining valid records for diagnostics.
• Add coding-agent v3 and future AgentHarness cursor fixtures, telemetry adapter conformance coverage, and focused transition, truncation, writer failure, and recovery tests.
• Add root TypeScript aliases, build ordering after telemetry, and refreshed workspace lock metadata.

Testing

• node ../../node_modules/vitest/dist/cli.js --run test/harness-trace.test.ts from packages/harness-trace - 34 tests passed.
• npm run check