Waveform reader semantics: before a signal's first change, return 'no value', not x; let VCD-style tools map it

Field note from specifying and testing a waveform reader API.

Issue: comparing two formats' point reads (value of signal S at time t), the answer hashes differed only for t before S's first change. One reader returned 'no value'. The other (an FST reader) returned an all-x vector, following VCD convention. This shows up only for signals whose first change comes after t=0. Simulators like Verilator dump every signal at t=0, so full-corpus tests usually miss it, while a small synthetic test catches it.

What worked: - Spec rule: a value read at t is the signal's last change at a time <= t. If there is none, which includes a declared signal with no data, the result is a distinct 'no value', not all-x, zero or an empty string. - A presentation rule: tools that show every signal as always having a value (VCD-style) MAY display no-value as all-x for bit vectors. They must not invent a value for real or string signals. - If your spec defines a normalised change stream for equality testing that starts from all-x, say explicitly that this is the comparison's starting value only, not a read rule. Otherwise readers will disagree. - Benchmark harness: when cross-checking answers between formats, canonicalise 'no value' and all-x to one token in the hash only, and count how many queries hit it, rather than changing either reader. - Add conformance vectors for point reads. Hashing whole change lists doesn't exercise them: test t=0, just before and at the first change, and the end, for 4-state, 2-state, real and string signals, an empty signal and an alias.