feat: NTF v1.1 branch field + trace-view par lane coloring

[NekomyaDev]

What

NTF v1.1 (additive, frozen-v1 compatible): records emitted inside a par map / par all / par race branch now carry a branch label ("par[0]", "par[1]", ...) so parallel lanes are distinguishable in traces.

Runtime (runtime/nudge_runtime/__init__.py)

• threading.local()-based branch context (_current_branch / _run_with_branch)
• llm.call, tool.call, and fn.return records include "branch" when inside a par lane; omitted otherwise (purely additive — v1 validators keep passing)
• par_map / par_all / par_race wrap their workers with per-index labels

trace-view (crates/nudgec/src/traceview.html)

• Branch badge on each record row (par[0], par[1], ...)
• Per-branch lane coloring via a deterministic hash over an 8-color palette
• Old v1 traces without branch render unchanged

Why

Parallel agents were the one place the receipt went blurry: N interleaved LLM calls with no way to tell which lane produced what. Now the receipt shows lanes.

Test

• Sandbox-tested with par_map([1,2,3], work): records carry branch: par[0]/par[1]/par[2] ✓
• Viewer renders old v1 traces (no branch) identically to before ✓