docs: document SRJ export behavior and trace-width limitations - #1023

tsci export -f srj is absent from the export reference, and the current build path can run autorouting before writing the file and lose the board's preferred trace width. This documents the command, the distinction between minimum and nominal widths, and the limitations users should check when sharing routing input.

Motivation

Item 27 of Jan Mrázek's feedback email to Severin Ibarluzea, dated November 9, 2026, reports an approximately 90-second SRJ export on his board and connections receiving minTraceWidth instead of defaultTraceWidth. The report used tsci 0.0.3127 and @tscircuit/core 0.0.2311; the timing is reported feedback, not a benchmark performed for this PR. The email also describes agents leaving the tscircuit workflow to build custom tools. Documenting the supported export workflow and its limitations gives users and agents a clearer starting point for debugging within tscircuit.

Changes

• Add SRJ to the supported formats and show TSX and Circuit JSON export commands.
• Explain minimum, board-default, and per-connection nominal widths with a 0.10 mm minimum, 0.35 mm default, and 0.55 mm explicit trace example.
• Describe full-build/cache behavior, lost width metadata, and the information needed for an actionable export report.

Why this comes before the implementation

The documentation establishes the expected behavior for follow-up PRs:

1. circuit-json: add optional pcb_board.nominal_trace_width to the canonical schema/type and verify unit-normalized parsing preserves it independently of min_trace_width.
2. core: emit and restore the field and use it when regenerating SRJ, preserving explicit connection widths and each connection's owning board default.
3. cli: export before routing the selected parent scope, preserve routed child copper, and keep partial exports separate from the full-build cache. The CLI delay can also be fixed independently of the schema work.

This PR changes documentation only; it describes the current limitations without claiming the runtime fixes are available.

Validation

• bun run build
• bun run typecheck
• bun run format:check --vcs-enabled=true --vcs-client-kind=git --vcs-use-ignore-file=true (excludes generated build files)
• git diff --check