docs: define trajectory architecture and implementation plan

[minixalpha]

Summary

• Define Source Record, Native Event, Journal Entry, Event Journal, and ATIF Trajectory as distinct layers in the project vocabulary.
• Add bilingual research comparing Pi, Claude Code, Codex, OpenCode, and Grok event surfaces, including reproducible local captures and schema-faithful ATIF mappings.
• Document OpenRouter actual-cost accounting, generation-based reconciliation, the unified Chat Completions transport, and the provider-neutral boundary nano will need.
• Replace the earlier public native-trajectory proposal with an internal append-only Event Journal and public ATIF-v1.7 output.
• Add the 0.8.x implementation sequence for events, journal persistence, cost enrichment, ATIF projection, CLI integration, Harbor collection, and layered validation.
• Explicitly defer --output-format and stream-json to a later PR while preserving the shared runtime-fact model.

Architecture decisions

• Core emits agent-independent Native Events; the journal writer adds run identity, strict sequence, and record time to form durable Journal Entries.
• --trajectory PATH exposes only an ATIF-v1.7 snapshot. The internal Event Journal remains replayable implementation state rather than a second public trajectory format.
• Cost retains provenance. Provider-reported OpenRouter values may populate standard ATIF cost fields, while calculated, partial, or unknown values remain explicitly labeled.
• A future OpenRouter transport should default to Chat Completions; the existing Anthropic Messages compatibility path can reconcile missing cost through generation IDs.
• Harbor passes a trajectory path and reads ATIF instead of translating a native trajectory itself.

Validation

• Confirmed all three new English research notes match the heading structure and line count of their Chinese sources.
• Regenerated the complete English 0.8.x development notes from the Chinese source.
• Checked the new English documents for Chinese-text residue, balanced code fences, and existing relative-link targets.
• Ran git diff --check successfully.

Documentation only; no runtime behavior changes.