Add nanosecond timestamp support

[KavanPrice]

Summary

Adds end-to-end nanosecond timestamp support throughout the ACS data pipeline, from edge agent to InfluxDB. Implements Option B from issue #652, repurposing the existing Sparkplug uint64 timestamp field to carry nanoseconds rather than adding a new metric.

A value-check heuristic (< 10¹⁵ = milliseconds, ≥ 10¹⁵ = nanoseconds) ensures backward compatibility during the transition period where old and new edge agents coexist.

• lib/js-service-client - decoder populates timestampNs: bigint alongside timestamp: Date on all decoded Sparkplug packets; existing consumers are unaffected
• acs-edge - encodes outgoing timestamps as nanoseconds; auto-stamps all incoming driver data at nanosecond precision via process.hrtime.bigint(); extracts timestampNs from JSON driver payloads
• uns-ingester-sparkplug - modifies UNS JSON payloads to reflect nanosecond precision in timestamp
• historian-sparkplug - writes nanosecond timestamps to InfluxDB at ns precision
• historian-uns - reads timestamp from UNS payloads and writes to InfluxDB at ns precision

Driver usage

No changes required for existing drivers. To supply a source nanosecond timestamp from a new driver, include timestampNs as a numeric string in the JSON payload:

{ "value": 123.45, "timestampNs": "1779983216100923659" }

If absent, the edge agent automatically stamps values using process.hrtime.bigint().

How to test

1. Deploy updated images for acs-edge, uns-ingester-sparkplug, historian-sparkplug, historian-uns
2. Subscribe to UNS/v1/# - payloads should include a modified timestamp that now has nanosecond precision
3. Query InfluxDB with Flux, adding map(fn: (r) => ({r with ns: string(v: uint(v: r._time))})) - verify sub-millisecond digits are non-zero
4. Confirm existing millisecond-only drivers continue to work (no timestamp breakage)

Known gaps

• Tests in acs-edge/tests/lib/helpers/typeHandler.test.ts need updating for new timestampNs parameter in setValueBy* signatures
• Individual driver updates to surface actual source nanosecond timestamps are deferred
• Subscription-based drivers using emit('data', ...) with non-JSON payloads fall back to now_ns() auto-stamping

Closes #652