Admit an event instant and the signal-to-noise ratio and level pair

[Simon-McIntosh]

Admits three physical_base tokens the WEST standard-name batch needs, and settles a unit question by splitting rather than choosing.

event_instant

A scalar for the time-coordinate value at which a discrete event occurs. The consumer is the Data Dictionary path summary/disruption/time/value, which had no admissible base.

Three meanings have to stay distinct, and the definition says which one this denotes and which two it excludes: the instant an event occurs, an elapsed duration, and the sampled time axis. A token conflating them composes onto the wrong sources.

The grammar was probed first to check whether it could already express an instant. It can parse instantaneous_time as qualifier instantaneous plus base time, while event_time, occurrence_time and event_instant were all rejected. That spelling was judged not to carry event-instant semantics: base time is the coordinate axis, so instantaneous_time reads as the axis sampled instantaneously rather than the instant an event occurred. Hence a distinct base.

signal_to_noise_ratio and signal_to_noise_level

One prior note recorded signal-to-noise as decibels; a consuming plan described it as dimensionless. Those are different admissions with different composition consequences, so both are admitted with the distinction made explicit:

Token	Unit	Meaning
signal_to_noise_ratio	1	the linear power quotient
signal_to_noise_level	dB	its logarithmic representation

Collapsing them would either lose the decibel form diagnostics actually report, or hang a decibel unit on a ratio. The dimensionless unit is the string 1, not an absent unit — a null unit is a different and wrong state.

Verification

• Focused suite: 123 passed, 28 skipped, 5 xfailed at baseline and again after each admission.
• Each token read back from the built vocabulary rather than from the edited source, because a spec and its generated snapshot have diverged in this repository before.
• One composed example name per token parsed strictly and round-tripped byte-identically.
• build-grammar --check exits 0 with generated files in sync.

Intended to be followed by a stable patch release, v0.9.1, cut through the release CLI so the tag drives the version.